I may investigate this further in due course, as I know that VM will speed up a ppc under many circumstances. Only not yet....
The particular 8600 in question here had previously given me trouble with a 2 x 604e, 200 MHz card in it (though from a different install of MacOS 8.6), and as I was having trouble with the single card, I thought the machine itself was possibly toast and would need to be replaced/ tossed. Having found that I could make it run stably with a 1 x 604e card in there merely by the turning off of VM, last night I replaced the single processor with the dual processor card that had been sitting idle.
Bliss. I don't want to change anything for a while.
Day to day applications are, of course, for the most part not able to take much advantage of the second processor (multimedia apps often do, however). This doesn't bother me much, because my day to day applications don't need a second processor. The kernel in 8.6, however, was rewritten to be more multiprocessor friendly in general, and I gather that the Finder in particular took greater advantage of the second processor.
Now I happen to have a PowerBook 2400c/180, which has a 603ev processor with 80-odd MB of RAM. It has MacOS 8.6 on it, and it is sitting alongside the dual 604e 8600/ 200, which also has MacOS 8.6 on it. I use the 2400c on a regular basis, so I know it well. Both machines have a 256 L2 cache, and both are of an era. However, if I try to do something like print or to copy a file on the 2400c, I'm effectively unable to do anything else until the first job is processed, the multitasking is so painfully poor. I have a lot of experience of the 604e and 604ev, and have always found them to be far better, and indeed, I prefer a 604 chip to a G3, in general. The dual 604e, however, is a revelation. Yes, there are inherent limits in the OS, but the dual 604e performs like I have never seen a classic Mac perform when doing several things at once.
I could make this my main machine....