There are 5 usual possible slowdown causes:
- bad powersaving driver, it causes timer that WinUAE uses to return bogus values. Basically AMD-only issue, make sure bios is latest and Cool'n'Quiet driver is up to date, or disabled.
- CPU Idle setting in CPU-panel set too high. 1 or 2 step from left is good test setting.
- Priority in priority-panel set to High. In some cases Normal can cause similar problems, usually only when CPU Idle is disabled. Try "Low". (low does not mean low emulation speed)
- some realtime virusscanners can slowdown harddrive emulation (can also be related to priority setting)
- vsync, make sure vsync in display panel is disabled.
First 3 settings are very PC/Windows/driver version specific.