A 1.24 Ghz G4 with 512MB of RAM should not be too slow. Granted it will not match a G5 but that's obvious.
When was the last time you ran any form of utility on this Mini? Try Disk Utility and repair permissions and you could also try Onyx which is a free utility that is very helpful.
Disk Warrior and Tech Tool Pro are two "must haves" for any Mac owner.
Try the first two, free, things first and let us know what happens

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