First of all, Avalon, RAM and even possibly hard drive space could also affect how well each of your computers handle. What's the RAM for the G4, and what's the RAM for the one with the Pentium 4? And what's the hard drive space for each, and how much of it has been taken up?
Now, COULD Apple make processors? Almost certainly, since while they have never actually done it entirely before, I'm pretty sure people from Apple actually have assisted people from IBM or Motorola in building the processors. Maybe I'm wrong, I don't know. But this would be a completely new field for Apple. A completely new gamble. They would not likely be able to compete with the other companies. This would be incredibly risky, and it would be a mistake.
But probably the main reason is simply because Apple currently has a contract with IBM for them to be the ones to design the processors for the Macs. Apple also still has a bit of a contract left with Motorola, as there are still G4s out there still in production (like for PowerBooks, iBooks, and eMacs). Until these contracts expire, Apple's not gonna even think about making their own processors, and even when the contracts do expire, it's incredibly, incredibly unlikely (Apple almost certainly won't renew the contract with Motorola, even when it's time to make a G6).
If I made any factual errors above, then I apologize, and request that you correct my mistakes.