No speed win from ddply

gistfile1.txt

> system.time({a=by(raw, raw$Img, strip.tracing.dups); b=do.call(rbind,a)})
   user  system elapsed 
 35.127   5.569  40.398 
> system.time({a=by(raw, raw$Img, strip.tracing.dups); b=do.call(rbind,a)})
   user  system elapsed 
 35.559   5.482  40.728 
> system.time({a=by(raw, raw$Img, strip.tracing.dups); b=do.call(rbind,a)})
   user  system elapsed 
 35.666   4.975  40.366 
> system.time({a=ddply(raw, .(Img), strip.tracing.dups)})
   user  system elapsed 
 36.454   6.814  42.877 
> system.time({a=ddply(raw, .(Img), strip.tracing.dups)})
   user  system elapsed 
 37.629   7.832  45.064 
> system.time({a=ddply(raw, .(Img), strip.tracing.dups)})
   user  system elapsed 
 37.283   7.224  44.058