[Newbies] Speed

Absolute beginner playing with smalltalk. I have a simple program that opens a number of text files, plays with data closes files and gives a count of things in the files. (Its actually my Squeak version of the Cincom VW Tutorial). I have run it on Squeak, Pharo and Cuis images. What suprised me was the difference in speeds. I haven't done tried to time it (I should do that or at least work out how to) but Squeak and Pharo take about the same time whereas Cuis is significantly quicker. I know its a cut down image but the difference is remarkable. Is this to be expected?

John