I'll answer this a couple different ways.
First, the latter generations of G4, and the motherboards they were connected to, continue to give very respectable performance. On that basis, it's really nothing to worry about.
Second, let me give you some insight in terms of my own configuration, and this may further shed some light on your question and concerns.
I generally run my notebook in "Reduced" processor mode. I do this for two reasons: I don't like it generating that kind of heat, and when I use it as a "laptop", I want to conserve battery power as much as possible. So, what does this mean to you?
Basically the G4 CPUs can be run at one of two speeds; 100% and 50%. In real world terms, that means 1499 MHz (1.49 GHz) and 749 MHz (0.75 GHz).
So, like right now, my 1.5 GHz G4 is really running at the speed of an older Titanium PowerBook, and frankly system performance is practically identical, especially where user interaction is concerned.
About the only times I run it at full speed are when I'm fooling around with Garage Band, or if I'm messing around in a 3D game (and I don't do either all that often).
Even when I'm running Photoshop I have no issues running an effectively 749MHz PowerBook.
So take all of that for what you will.