Our problem is very slow drawing, which was unexpected for the BeOS. We don't want to change the automatically generated interp.c, so we need to fix this in one of the machine specific calls.
Don't forget that it is entirely possible that the 'copy a big bitmap from here to there on the screen' might not be as well optimised as other parts of the system.
But on most systems, displaying bitmaps is a crucial part of the graphics interface and usually pretty fast. You might check out different host display depths in combination with different Squeak display depths since a bad combination may dramatically affect performance (try using Squeak's 16bit display depth with a 6-5-5 or 5-6-5 host depth ;-)
Andreas