you can use timestretch() . The quality actually isn't that bad
http://avisynth.nl/index.php/TimeStretch
I hardcoded this example for 90% speed, for some reason you can't use variables to calculate in the changefps field, but the value would be 100/90 or 100/value , where value is expressed in percentage
you can check with info that the new a/v length is the same (or very close)
Code:
#Assuming a 23.976fps A/V source...
whateversource()
trim()
timestretch(tempo=90)
changefps(1.111111*last.framerate)
assumefps(24000,1001)
#info()
It would be more elegant to use variables, so you can tweak the speedup/slowdown more easily, but the "m" isn't calculated, not sure why here
Code:
#Assuming a 23.976fps A/V source...
p=90 #in percent
m=(100/p)
whateversource()
trim()
timestretch(tempo=90)
changefps(m*last.framerate)
assumefps(24000,1001)
#info()