Sounds like a configuration error - using software scrolling (no acceleration) and not using a shadow framebuffer. That results in a lot of reads from video memory to the CPU (not GPU), which is often that sort of very slow.
Try enabling some kind of acceleration at least :-)
If you can't, perhaps you're using the "vesa" driver, in which case try enabling the shadow framebuffer.
This isn't new to Xorg 1.5, but maybe it configures itself differently and gets this wrong.