I use Adobe's Reader and ThinkFree Office, but they both are too slow. Any suggestions?
Starting with Android 3.0 (Honeycomb), Google introduced hardware accelerated 2D rendering pipeline. For now it's a tablet-only feature, but hopefully we'll get that in the unified Ice Cream Sandwich platform.
That should be a bit with that slowness issue with PDF readers since they are only able to do software rendering in Gingerbread and older.
There is no hardware accelerated PDF reader for Android at the time I am writing this (correct/edit me if I am wrong).
As kernal already sad, the performance depends on your phone. You could try to downsize the images in the PDFs for better performance on your android device.
It seems like the better choice is ezPDF Reader. It's a paid app, but the screenshots look just great!
Remember the performance depends on which phone do you have... :-/