Looks like SFS and PFS require a 68020 so that rules them out, looks like Im stuck with FFS for now.
I wonder why it's so slow on my real miggy, using WinUAE and a 500+ HW profile (including 1mb chip & 8mb fast ram) it's very responsive by comparison.
This version of PFS doesn't require a 68020, and it's KS1.3 compatible (yes, has 68000 DirectSCSI version included):
http://aminet.net/package/disk/misc/pfs3ks13I use it on two A600's with 20GB Toshiba 2.5" drives, both 68000 and KS2.0 and it works brilliantly.
PFS eats more RAM than FFS, so I only have one Partition (my machines are only 2MB fast). Also, minimum amount of buffers required for PFS is 70, which is what I use and it's fine.

Backtracking a bit to your original problem, if a disk is validating then you'll just have to wait for it to complete, or it will never complete (may take ages depending on partition size). When your Workbench screen appears, select your partition that you suspect to be validating, and select Icon Information. It will tell you there under Disk State "Validating" or "Read/Write". If it's validating, let it complete. It's finally complete when the drive access light goes out (assuming your floppy drive light on your A500 light is wired as hard drive light as well).
Edit: Almost forgot to mention, validating a partition requires a certain amount of memory, dependant on partition size (larger partition = more memory required). I don't know the numbers involved unfortunately, I just remember reading about it. But yes, I'd move to PFS3 for sure.