Using Sass 3.2.0.alpha.11 (Bleeding Edge) on MacBookPro3,1 with Snow Leopard 10.6.8

sass-test-elseif-deep.scss

$a: #f00;
$b: #00F;

@if $a == a {}
@else{
   @if $a == b {}
   @else{ 
       @if $a == c {} 
       @else{
           @if $a == d {}
           @else {
               @if $a == e {}
               @else{
                   @if $a == f {}
                   @else{
                       @if $a == g {}
                       @else{
                           @if $a == h {}
                           @else{
                               @if $a == i {}
                               @else{
                                   @if $a == j {}
                                   @else{
                                       @if $a == k {}
                                       @else{
                                           @if $a == l {}
                                           @else{
                                               @if $a == m {}
                                               @else{
                                                   @if $a == n {}
                                                   @else{
                                                       @if $a == o {}
                                                       @else{
                                                           @if $a == p {}
                                                           @else{
                                                               @if $a == q {}
                                                               @else{
                                                                   @if $a == r {}
                                                                   @else{
                                                                       @if $a == s {}
                                                                       @else{
                                                                           @if $a == t {}
                                                                           @else{
                                                                               @if $a == u {}
                                                                               @else{
                                                                                   @if $a == v {}
                                                                                   @else{
                                                                                       @if $a == w {}
                                                                                       @else{
                                                                                           @if $a == x {}
                                                                                           @else{
                                                                                               @if $a == y {}
                                                                                               @else{
                                                                                                   @if $a == z {}
                                                                                               }
                                                                                           }
                                                                                       }
                                                                                   }
                                                                               }
                                                                           }
                                                                       }
                                                                   }
                                                               }
                                                           }
                                                       }
                                                   }
                                               }
                                           }
                                       }
                                   }
                               }
                           }
                       }
                   }
                       
               }
           }
       }
   }
}

sass-test-elseif-nested.scss

$a: #f00;
$b: #00F;

@if $a == a {}
@else{
   @if $a == b {}
   @if $a == c {}
   @if $a == d {}
   @if $a == e {}
   @if $a == f {}
   @if $a == g {}
   @if $a == h {}
   @if $a == i {}
   @if $a == j {}
   @if $a == k {}
   @if $a == l {}
   @if $a == n {}
   @if $a == o {}
   @if $a == p {}
   @if $a == q {}
   @if $a == r {}
   @if $a == s {}
   @if $a == t {}
   @if $a == u {}
   @if $a == v {}
   @if $a == w {}
   @if $a == x {}
   @if $a == y {}
   @if $a == z {}     
}

sass-test-elseif-parallel.scss

$a: #f00;
@if $a == a {}
@else if $a == b {}
@else if $a == c {}
@else if $a == d {}
@else if $a == e {}
@else if $a == f {}
@else if $a == g {}
@else if $a == h {}
@else if $a == i {}
@else if $a == j {}
@else if $a == k {}
@else if $a == l {}
@else if $a == n {}
@else if $a == o {}
@else if $a == p {}
@else if $a == q {}
@else if $a == r {}
@else if $a == s {}
@else if $a == t {}
@else if $a == u {}
@else if $a == v {}
@else if $a == w {}
@else if $a == x {}
@else if $a == y {}
@else if $a == z {}

time-command-output-1.txt

> time sass sass-test-elseif-parallel.scss

### I hit ctrl-c, was too long


real	24m35.363s
user	21m20.058s
sys	1m26.334s

time-command-output-2.txt

> time sass sass-test-elseif-nested.scss

real	0m0.294s
user	0m0.232s
sys	0m0.042s

time-command-output-3.txt

> time sass sass-test-elseif-deep.scss

### I hit ctrl-c, was too long

real	10m10.347s
user	8m1.981s
sys	0m40.503s