Descriptions:
WinUAE uses all host CPU power when active
- Fastest possible-mode uses all CPU power by design. Move CPU idle-slider in CPU-tab couple of steps right until CPU usage is under 30% but don't adjust too much or emulation performance will suffer.

Filesystem access is very slow, MIDI IN drops notes
- Set WinUAE's priority to normal or below normal

Recent WinUAE versions are slower than for example WinUAE 0.8.8.
- UAE's main goal is compatibility, not speed.
- many Amiga subsystems are now emulated (nearly) cycle-exactly in newer versions (copper, bitplanes, sprites and floppy). Cycle-exact emulation is usually more complex = slower.

WinUAE 0821 is much slower than 0817
- make sure your JIT cache and CPU speed settings are correct. Some old configuration files are not 100% compatible with 0821 and newer releases. (Usually JIT cache size resets to 0 and CPU speed to "Match A500 speed")

My CDROM drives are not anymore visible in 0821
- "Add PC Drives at Startup" does not anymore add any removable devices
- you need to install Amiga CDFS and tick "usescsi.device emulation"-checkbox in misc-page. Confirmed working CD filesystems are for example IdeFix97, AmiCDFS and CacheCDFS. Set device name to "uaescsi.device" and unit to 0. (some setups' unit number may not be zero, especially if you have scsi devices. You can enable debugging and look winuaetxt.log for correct unit numbers)

Refresh rate selection box only shows "Default" and VSync box is disabled
- your display driver doesn't support DirectDraw refresh rate selection.

Game X doesn't want to save/load to disk
- game probably uses non-standard save format, use "Custom Floppy" image instead of "Standard Floppy" for save disk.

MakeCD freezes at the end of burn with my IDE burner
- download latest version of MakeCD and add "MAKECD_ATAPI" and "MAKECD_NO_DCR" to MakeCD's icon tooltypes