Used Pester to do some speed tests, because the comments on my stackoverflow post (http://stackoverflow.com/a/16175967/615422) peaked my interest.

ForEachSpeeds.Tests.ps1

<#
Invoked the test with the following command, so that I could save the times take the average of each Context:

```posh
Invoke-Pester -OutputFile test.xml -OutputFormat NUnitXml
```

Then I did some math on the results as shown (yes, I use pipelines when not scripting):

```posh
[xml]$xml = Get-Content .\test.xml
$times = @(
    ($xml.'test-results'.'test-suite'.results.'test-suite'.results.'test-case' | ?{ [int]$_.description -le 1000 } | %{ [decimal]$_.time }),
    ($xml.'test-results'.'test-suite'.results.'test-suite'.results.'test-case' | ?{ ([int]$_.description -ge 1001) -and ([int]$_.description -le 2000) } | %{ [decimal]$_.time }),
    ($xml.'test-results'.'test-suite'.results.'test-suite'.results.'test-case' | ?{ [int]$_.description -ge 2001 } | %{ [decimal]$_.time })
)

# Get Averages
$avgs = $times | %{ $_ | Measure-Object -Average }
Write-Host "Averages: $($avgs | Out-String)"

# Get Medians
$meds = $times | %{ 
    $data = $_ | sort
    if ($data.count%2) {
        $MedianValue = $data[[math]::Floor($data.count/2)]
    } else {
        $MedianValue = ($data[$data.Count/2],$data[$data.count/2-1] |measure -Average).average
    }    
    $MedianValue
}
Write-Host "Medians: $($meds | Out-String)"

#Get Modes
$mods = $times | %{ '---'; ($_ | group | sort -Descending Count)[0..5] }
Write-Host "Modes: $($mods | Out-String)"
```

Results are in the comments.
#>

$computerSystem = Get-CimInstance CIM_ComputerSystem
$computerOS = Get-CimInstance CIM_OperatingSystem
$computerCPU = Get-CimInstance CIM_Processor

@"
Manufacturer: {0}
Model: {1}
CPU: {2}
RAM: {3:N2} GB
Operating System: {4}
Service Pack: {5}
PS Version Table: {6}
"@ -f (
    $computerSystem.Manufacturer,
    $computerSystem.Model,
    $computerCPU.Name,
    ($computerSystem.TotalPhysicalMemory/1GB),
    $computerOS.caption,
    $computerOS.ServicePackMajorVersion,
    ($PSVersionTable | Out-String)
)

Describe 'Looping Speed Test' {
    BeforeEach {
        $hash = @{
            a = 1
            b = 2
            c = 3
        }
    }

    Context 'Pipeline Test' {
        foreach ($i in 1..500) {
            It $i {
                {
                    $hash.Keys | % { "key = $_ , value = " + $hash.Item($_) }
                } | Should Not Throw
            }
        }
    }

    Context 'ForEach GetEnumerator Test' {
        foreach ($i in 1001..1500) {
            It $i {
                {
                    foreach ($h in $hash.GetEnumerator()) {
                        "$($h.Name): $($h.Value)"
                    }
                } | Should Not Throw 
            }
        }
    }

    Context 'ForEach Keys Test' {
        foreach ($i in 2001..2500) {
            It $i {
                {
                    foreach ($h in $hash.Keys) {
                        "${h}: $($hash.Item($h))"
                    }
                } | Should Not Throw 
            }
        }
    }
}

Here's my computer information:

Manufacturer: Dell Inc.
Model: Latitude E7240
CPU: Intel(R) Core(TM) i7-4600U CPU @ 2.10GHz
RAM: 7.94 GB
Operating System: Microsoft Windows 8.1 Enterprise
Service Pack: 0
PS Version Table:
Name                           Value
----                           -----
PSVersion                      4.0
WSManStackVersion              3.0
SerializationVersion           1.1.0.1
CLRVersion                     4.0.30319.34209
BuildVersion                   6.3.9600.17400
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0}
PSRemotingProtocolVersion      2.2

Here's my Averages, Medians, and Modes (calculated as shown above, in the comment block):

Averages:

Count    : 500
Average  : 0.0520342
Sum      :
Maximum  :
Minimum  :
Property :

Count    : 500
Average  : 0.049086
Sum      :
Maximum  :
Minimum  :
Property :

Count    : 500
Average  : 0.0498152
Sum      :
Maximum  :
Minimum  :
Property :

Medians: 0.049
0.0492
0.0498

Modes: ---

Count Name                      Group
----- ----                      -----
   15 0.0492                    {0.0492, 0.0492, 0.0492, 0.0492...}
   15 0.0498                    {0.0498, 0.0498, 0.0498, 0.0498...}
   14 0.0497                    {0.0497, 0.0497, 0.0497, 0.0497...}
   14 0.0501                    {0.0501, 0.0501, 0.0501, 0.0501...}
   13 0.0499                    {0.0499, 0.0499, 0.0499, 0.0499...}
   13 0.0487                    {0.0487, 0.0487, 0.0487, 0.0487...}

---
   21 0.0489                    {0.0489, 0.0489, 0.0489, 0.0489...}
   20 0.0498                    {0.0498, 0.0498, 0.0498, 0.0498...}
   17 0.0493                    {0.0493, 0.0493, 0.0493, 0.0493...}
   16 0.0492                    {0.0492, 0.0492, 0.0492, 0.0492...}
   15 0.0501                    {0.0501, 0.0501, 0.0501, 0.0501...}
   15 0.0502                    {0.0502, 0.0502, 0.0502, 0.0502...}

---
   22 0.0505                    {0.0505, 0.0505, 0.0505, 0.0505...}
   17 0.0496                    {0.0496, 0.0496, 0.0496, 0.0496...}
   17 0.0502                    {0.0502, 0.0502, 0.0502, 0.0502...}
   16 0.0494                    {0.0494, 0.0494, 0.0494, 0.0494...}
   16 0.0498                    {0.0498, 0.0498, 0.0498, 0.0498...}
   15 0.0501                    {0.0501, 0.0501, 0.0501, 0.0501...}

Note: The first calculation is the Pipeline Test, then the ForEach GetEnumerator Test, then the ForEach Keys Test.

In case you want my values, here's the result of $times | ConvertTo-Json -Compress:

[{"value":[1.4995,0.0445,0.0484,0.0492,0.0471,0.0493,0.0492,0.0457,0.0504,0.049,0.0559,0.0471,0.0473,0.0432,0.0508,0.0496,0.0446,0.0488,0.0487,0.0498,0.0483,0.0498,0.0482,0.0488,0.0481,0.0482,0.0487,0.0493,0.0464,0.0482,0.0441,0.05,0.0648,0.0519,0.0485,0.0501,0.049,0.0483,0.0442,0.0499,0.0512,0.0492,0.0479,0.0499,0.0481,0.0488,0.0472,0.0486,0.0485,0.0499,0.0493,0.049,0.0505,0.0497,0.0483,0.0503,0.0468,0.0498,0.0493,0.0542,0.045,0.0498,0.0479,0.0472,0.048,0.0484,0.0513,0.0492,0.0496,0.0558,0.0476,0.0487,0.0443,0.0481,0.0481,0.0501,0.0531,0.0491,0.0489,0.0487,0.049,0.0474,0.0493,0.0472,0.0489,0.048,0.0499,0.0485,0.0452,0.0493,0.0487,0.0444,0.0499,0.0472,0.0502,0.0474,0.0525,0.0493,0.0498,0.0512,0.0497,0.0508,0.0487,0.0485,0.0506,0.0496,0.0488,0.0431,0.0483,0.0494,0.0535,0.0497,0.0489,0.0514,0.0506,0.0501,0.0495,0.0497,0.0449,0.0512,0.0492,0.0501,0.0451,0.0482,0.0478,0.0491,0.0493,0.0503,0.053,0.0499,0.0533,0.048,0.0495,0.0492,0.0503,0.0515,0.0456,0.0468,0.0488,0.0484,0.0492,0.0473,0.0486,0.0476,0.0481,0.05,0.0505,0.0497,0.0447,0.0497,0.0499,0.0515,0.0505,0.0501,0.0514,0.0462,0.0498,0.052,0.0497,0.0498,0.0488,0.0478,0.0444,0.0465,0.046,0.0498,0.0492,0.0454,0.0471,0.0516,0.0411,0.0478,0.0507,0.05,0.0455,0.0521,0.0558,0.0488,0.0503,0.0474,0.0455,0.0485,0.0507,0.0536,0.0488,0.0483,0.0453,0.0471,0.0499,0.0487,0.0513,0.0502,0.0499,0.0487,0.0485,0.048,0.0488,0.0489,0.0464,0.0512,0.0615,0.0498,0.04,0.0473,0.0453,0.0461,0.0486,0.0523,0.0459,0.046,0.0499,0.0477,0.0467,0.0502,0.0484,0.0501,0.0521,0.049,0.0503,0.0468,0.0497,0.0489,0.0486,0.0481,0.0459,0.0462,0.0483,0.0495,0.0504,0.0489,0.0502,0.0502,0.0457,0.0497,0.0503,0.0506,0.0475,0.0471,0.0445,0.0485,0.0469,0.049,0.0503,0.0498,0.0505,0.0513,0.0492,0.0498,0.0508,0.0515,0.0499,0.0464,0.0508,0.0511,0.0468,0.0484,0.05,0.0484,0.0494,0.0506,0.0508,0.0504,0.049,0.0496,0.0506,0.05,0.0456,0.0515,0.0492,0.0494,0.0523,0.0491,0.0496,0.0505,0.0522,0.0492,0.0501,0.0488,0.049,0.0497,0.0512,0.0492,0.0433,0.0506,0.0461,0.0499,0.0498,0.0473,0.0453,0.0479,0.0503,0.0498,0.0511,0.0491,0.0456,0.0508,0.0523,0.0438,0.048,0.0524,0.0514,0.0477,0.0492,0.046,0.0482,0.0494,0.0508,0.0478,0.0475,0.0456,0.0477,0.0484,0.0487,0.0526,0.0529,0.0464,0.0464,0.0453,0.0468,0.0556,0.0521,0.0461,0.0483,0.0477,0.0511,0.0462,0.0466,0.0532,0.0468,0.0476,0.0507,0.0464,0.0524,0.0468,0.0508,0.0483,0.046,0.0477,0.0443,0.0498,0.0469,0.0465,0.0517,0.0423,0.0468,0.0446,0.0513,0.0476,0.0436,0.0469,0.0459,0.0455,0.0429,0.0454,0.0464,0.0484,0.0548,0.0598,0.052,0.0434,0.0488,0.0459,0.0493,0.0477,0.0502,0.0453,0.0458,0.0664,0.0612,0.0535,0.0477,0.0478,0.0468,0.0581,0.0522,0.0505,0.0562,0.0463,0.0457,0.0429,0.0518,0.0461,0.0499,0.0463,0.0482,0.0453,0.0473,0.0474,0.0446,0.0446,0.0465,0.0457,0.0452,0.047,0.0502,0.0747,0.0497,0.0505,0.0464,0.0448,0.0436,0.0451,0.0463,0.0442,0.0507,0.0527,0.0559,0.0569,0.0495,0.0427,0.0535,0.0503,0.0485,0.0513,0.0566,0.0503,0.0496,0.0494,0.0494,0.0481,0.0497,0.0479,0.0715,0.0484,0.0501,0.0487,0.0484,0.0465,0.0483,0.053,0.0472,0.0489,0.0505,0.0492,0.0471,0.0531,0.0501,0.0487,0.0472,0.0487,0.0479,0.0501,0.0542,0.044,0.0517,0.0522,0.0461,0.0493,0.0496,0.057,0.0578,0.0481,0.0502,0.0487,0.0469,0.0492,0.0505,0.0479,0.0488,0.0453,0.0481,0.0497,0.0486,0.051,0.0497,0.046,0.049,0.0504,0.0495,0.0507,0.0501,0.0485,0.049,0.0447,0.0482,0.0485,0.0495,0.0669,0.0501,0.0512,0.0488,0.0515,0.0498,0.0483,0.0512,0.0451,0.0516,0.0503,0.0495,0.0493,0.0503,0.0501,0.0505,0.0493,0.0515,0.05,0.0459,0.0536,0.0501,0.0544],"Count":500},{"value":[0.1542,0.0492,0.0597,0.0487,0.0508,0.0549,0.048,0.0445,0.0493,0.0492,0.05,0.0515,0.0502,0.0495,0.052,0.052,0.047,0.0501,0.0497,0.0484,0.0495,0.0464,0.0447,0.0505,0.0498,0.0467,0.0483,0.0519,0.0494,0.0631,0.0478,0.052,0.0531,0.0483,0.0477,0.05,0.0494,0.0486,0.0481,0.0497,0.0516,0.0493,0.0482,0.0494,0.0485,0.0491,0.0416,0.0518,0.048,0.0513,0.0491,0.049,0.0512,0.0492,0.0471,0.0454,0.0492,0.0472,0.0489,0.045,0.0497,0.0457,0.0467,0.0491,0.0494,0.0473,0.0495,0.0493,0.0489,0.0492,0.0458,0.0504,0.0519,0.0511,0.0513,0.0498,0.0498,0.0497,0.047,0.0477,0.0475,0.0453,0.0502,0.0503,0.0442,0.0441,0.0504,0.0501,0.0489,0.0496,0.0483,0.0481,0.0493,0.0488,0.0502,0.0424,0.0511,0.0489,0.0464,0.0501,0.0477,0.0489,0.0501,0.0501,0.049,0.0468,0.0458,0.051,0.0493,0.0478,0.0485,0.0492,0.0457,0.0493,0.0476,0.0496,0.0492,0.0506,0.0494,0.0501,0.0458,0.0503,0.0465,0.0496,0.0483,0.0489,0.0499,0.0495,0.0494,0.051,0.0482,0.0509,0.0443,0.0475,0.0517,0.0481,0.05,0.053,0.049,0.0507,0.0502,0.0492,0.05,0.0499,0.0459,0.0487,0.0496,0.0492,0.0454,0.0504,0.0492,0.049,0.0497,0.0452,0.0495,0.0517,0.0451,0.0514,0.0472,0.0477,0.0506,0.0488,0.0485,0.05,0.0485,0.0519,0.0473,0.0491,0.0483,0.0482,0.0492,0.045,0.0489,0.0491,0.0478,0.046,0.0506,0.0496,0.054,0.0491,0.0505,0.0458,0.0483,0.0492,0.0484,0.0492,0.0482,0.0498,0.0501,0.0485,0.0488,0.0491,0.0497,0.0442,0.0486,0.0501,0.0451,0.0489,0.0497,0.0505,0.0484,0.0487,0.05,0.0452,0.0505,0.0488,0.048,0.0508,0.0495,0.0485,0.0517,0.0492,0.0514,0.051,0.0494,0.0487,0.0415,0.0475,0.048,0.0544,0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