pg 13 has info for pine plywood (soft) so instead of doing the math, just look at the table. Most of them show 18000 rpm as a reasonable speed so you could start there or you could go through the math just using the chip load numbers they give such as 0.004-0.006 for the plywood.
I am unsure if that chip load they provided is relevant for our system, but it looks like you can run super slow on the spindle and be ok. Please check the math and read the paper for verification, but too slow doesn’t appear to be the issue here.
I have been off the forum for awhile, but have had pretty good success with my Maslow set-up (after A LOT of small and large upgrades, adjustments and calibrations). Re: cutting rpm, bit size, material, cutting depth, and feed rate play into this. I like the math approach suggested by @orob, but I don’t think our routers go that slow (I’m away from my shop right now, so I cannot check my Rigid). Early on I thought that faster was better, and ran pretty much full bore, 18-20K rpm. And burned a lot of bits. So, one day I decided to see just how slow I could go and still get decent cuts. I use 1/4" for general cutting, and 1/8" bits when the cuts are complex, mostly 2 flute up-cutting, and cut mostly 1/4", 1/2" and 3/4" ply. Feed rate out of the box is pretty slow (and I do not recommend trying to go faster, as the use of chains and gravity are not friendly to speed). My cutting depths usually are around .15" on 1/4" ply (2 passes), .18 on 1/2" ply (3 passes), and .26 on 3/4" ply (all with a 1/4" foam insulation waste board underneath the workpiece). I think I settled on the following: 1/8" @ 11,000 rpm, and the 1/4" @ 13,000. At these speeds the bits do not “blacken”.
Patience and Persaverance rule in this Maslow world, so doing test runs is what it is all about.
Another thing I’d recommend is to get some spray on engine cleaner / degreaser and use that every 1 to 2 hours on your bits and wipe off the residue buildup. It really helps prolong the life of your bits, try it on some of your blackened bits and see how they perform.