Seems like you were using reasonable settings. You could probably increase your stepover if you want. I would run 70% of the tool diameter with no issues on my stock Shapeoko 3 and trim router with no problems (with single flutes). Also, are you conventional or climb cutting?
Climb milling 20 chars …
CORRECTION - That was a 1/4 278Z bit I was stating, not the 1/8 274z
I edited the previous post as well