1. A 5300 is going to be slow for many reasons, one of which is the hardware: the machine is essentially a PPC603e bolted onto an archaic logic board design. There is also no L2 cache. I have a couple of them, and know the story.

2. A 5300 running MacOS7.6 will be slow partly because because the MacOS was not then (nor ever was, truth be told) successfully updated from top to tail to RISC code. As late as 7.6, MacOS ran largely in emulation. The company's travails over the MacOS through the mid-90s are well documented, the problem being the move to RISC on the one side and the inability to ship a genuinely new OS for it on the other. The utility Speed Doubler can help here.

3. System 6 on almost anything that can run it is very fast. On an SE/30 is is a fast system. System 6 on a IIci is like greased lightning. I haven't had the pleasure on a IIfx, but that must be rather amazing.

4. A fast 68k machine running the right software is as fast or faster than a PPC for many purposes, with the possible exception of things such as floating point performance in Photoshop etc.

5. Something like BeOS, had it been adopted by Apple, would have allowed the PPC to shine in its true light. The MacOS did not.

6. Welcome to the 68k and early MacOS fanclub. This is partly why we are here, isn't it?