Did WB 3.5 have a Workbench prefs editor in the Preferences folder? You may want to have a look there. There are some options about how to render icons. Maybe moving the icons to fast memory will help. Generally speaking, my 3.9 system is dog-slow without EXEC being in fast memory as well. If you have an MMU or certain makes of accelerator, you can relocate exec into fast. The default is chip memory which is really slow memory because it's shared with all custom chips.
Hope this helps!