>> even on a PRS Standard, those speeds and feeds sound slow.
Sounds like the machine is worn out. Slowing things down reduces the forces on the end mill and should reduce backlash in the system. Cutting both the depth of cut and speed by half should yield about the same chip load and reduce the forces acting on the machine by a factor of four. Naturally, adjust the RPM's to compensate and get the best cut quality.
ShopBot Details:
2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool