With Win98, when you allow the default swap file behavior of letting Windows choose how to expand and contract, Windows will allocate about 50% of CPU resources when managing the size. Plus, you've got the hard drive time, PLUS if your swap file isn't contiguous, Windows will have to manage WHERE the free space is to put the swap file. (this was fixed starting with Win2000.)
Your speed improvement by setting the swap file size from 800M to 300M is very interesting to me! Wish I'd known this when I was still using that OS.
You might want to set your Win98 swap file to 300M min / 500M max and retest. This is the same settings as 300M as you already have, except it will expand to a max of 500M *only* if needed!