I had several AMD 486DX 100s in the herd. They actually ran RC5 at about the same rate as an Intel 486DX2-66.
I also had a Power Stacker 133 based on an AMD 586-133 chip. It was equally disapointing in RC5 performance.
Strangely, I had an AMD 486DX-66 that OCed to 80mhz just fine, and cracked RC5 just like an Intel.
viz
edit-It looks as though NWM and I had differing mileage.
When you consider that this was during a time when motherboard performance varied greatly. Hence my favorable performance with the AMD 486-66 in a Digital Equipment machine, and unfavorable in the AST 486-100.