Will take a source and destination directory then spawn a 7za.exe job for each directory. These are enqueued and executed in PowerShell jobs with a throttle to ensure only a limited number of compression jobs happen at any given time.

dir_compressor.ps1

$queue = New-Object System.Collections.Queue
$wait_state = 2
$job_count = 0
$completed_request = @{}
$job_dispached = @{}

$throttle = 3
$source_dir = "D:\"
$dest_dir = "E:\"

function Make-UniquePath {
    param([string]$path)

    $ptemp = $path
    $inc = 1;
    $ext = [IO.Path]::GetExtension($path)
    while (Test-Path $ptemp) {
        $tmp_ext = [String]::Concat("-",$inc.ToString("D2"),$ext)
        if ([String]::IsNullOrEmpty($ext)) {
            $ptemp = [String]::Concat($path,$tmp_ext)
        } else {
            $ptemp = $path.Replace($ext,$tmp_ext)
        }
        $inc++
    }
    return $ptemp
}


$xdir_file = "xdir_", $([Guid]::NewGuid()), ".txt" -join ""
$xdir_file = Join-Path $env:temp $xdir_file
gci $source_dir -Directory -Hidden | % { "`"$($_.FullName)`"" } | out-file $xdir_file -Encoding ascii
gci $source_dir -Directory | % { "`"$($_.FullName)`"" } | out-file $xdir_file -Encoding ascii -Append

$task_list = @()
$task_list += {
    $dest_file = Make-UniquePath "$dest_dir\_root.7z"
    New-Object PSObject -Property @{
                                    "Name"="_root";
                                    "Command"="& 7za a `"$dest_file`" `"$source_dir`" `"@$xdir_file`"";
                                    "Path"=$source_dir;
                                    }
    }.Invoke()
    
$task_list += gci $source_dir -Directory `
    | % { 
            $dest_file = Make-UniquePath "$dest_dir\$($_.Name).7z"
            New-Object PSObject -Property @{
                                        "Name"="$($_.Name).7z";
                                        "Command"="& 7za a `"$dest_file`" `"$source_dir\$($_.Name)`"";
                                        "Path"="$($_.FullName)";
                                        } }

$task_list | % { $queue.Enqueue($_) }

while (-not (($queue.Count -eq 0) -and ($job_count -eq 0))) {

    ## Collect complete jobs
    if ($job_count -gt 0) {
        Get-Job | ? { $_.Name.StartsWith("JOB-") } | ? { $_.State -eq "Completed" } | % {
            Write-Host "Job completed: $($_.Name)" -ForegroundColor Green
            $_ | Receive-Job
            $_ | Remove-Job
            --$job_count
            $completed_request.Add($_.Name, "Done")
        }
    }


    ## Spawn jobs
    while (($queue.Count -gt 0) -and ($job_count -lt $throttle)) {
        $task = $queue.Dequeue()
        $key = $task.Name
        $cmd = $task.Command
        if ($job_dispached.ContainsKey($key)) {
            ## Job already dispatched, avoid race where job is enqueued again before first job completed
            Write-Warning "Already dispatched: JOB-$target"
            continue
        }

        Write-Host "Job starting : JOB-$($key)" -ForegroundColor Cyan
        
        Start-Job -Name "JOB-$key" -ScriptBlock {param($cmd) $sb = [ScriptBlock]::Create($cmd); $sb.Invoke() } -ArgumentList @($cmd) | Out-Null
        $job_dispached.Add($key,"")
        ++$job_count
    }

    Write-Debug "No more requests this round, waiting $wait_state seconds before checking again"
    Start-Sleep -Seconds $wait_state    
}