The answer to that is in Activity Monitor. With it open, try a sample track to see how much processing power is being used while you're also working other applications, eg. browser.
A G4 never went past 1.42 GHz, and if you're running hungry applications, eg. Flash-related, then audio conversion might well struggle. I've not noticed lag or crash with Sound Converter with this 1.25 eMac, although it can take a long time to go, for example, from a large .aifc to .mp3.