Sharing my experience in case it helps someone else.
Material: HDPE - “King StarBoard Marine Board” brand in 0.490" thickness
Operations: full thickness slotting, full thickness pocketing
Here are the feeds and speeds that worked for me:
0.125" dia conventional upcut endmill, two flute (an single flute O-flute is often recommended but I didn’t have one of those)
100 IPM @ 0.08" DOC (probably could go both faster AND deeper - provided can clear chips)
spindle speed ~11K rpm (even slower normally better with HDPE, but it seemed like I had a harmonic around 10K)
plunge cuts at 30 IPM with full retract (to clear chips; probably could plunge faster)
conventional cut (NOT climb cut)
NO FINISH cut necessary to achieve good surface
…and the big secret: blow compressed air at the bit to aggressively clear chips. A dust shroud was simply inadequate for clearing chips in the slots, leading to chatter and chip rewelding onto the base material.