Originally: revno 104

echo '(length (s-head 10000000 s))' | time ikarus streams.ss 
Ikarus Scheme (Build 2006-12-06)
Copyright (c) 2006 Abdulaziz Ghuloum

> 10000000
> 
        4.97 real         3.46 user         1.50 sys

After optimization: revno 105
echo '(length (s-head 10000000 s))' | time ikarus streams.ss 
Ikarus Scheme (Build 2006-12-06)
Copyright (c) 2006 Abdulaziz Ghuloum

> 10000000
> 
        4.23 real         3.23 user         1.00 sys

As of r127:
        3.79 real         3.15 user         0.63 sys

With engines (r139):     
        3.94 real         3.23 user         0.69 sys
  Note: this is about 2% overhead.