I don't have a 1100M, but I can answer based on earlier machines:
A four-flute isn't generally recommended for aluminum, as the metal expands a bit when cut and will have trouble clearing the flutes. Three-flute is where it's at for aluminum. Also, I assume you're using carbide -- 7500 rpm 3/8" end mills is too fast in 6061 for HSS.
That being said, you should be able to do 0.035" WOC at 1/2" DOC with about 30 ipm feed rate on a 3-flute, assuming you use "adaptive clearing" in Fusion on an 1100.
How do you know the motor is "working hard"? Do you have the load meter? What does it show? Perhaps the PID controller in the VFD is tuned to not over-compensate, and thus it sounds a bit "chuggy" when first engaging? (The Adaptive Clearing toolpaths are great at improving this, btw. Make sure to set the no-load feedrate to something high like 200 in the linking settings.)
That finish doesn't look that good. It looks like you're either feeding faster or spinning slower than you indicate above. Maybe the extremely thin 0.010" cut size is the problem?
Or there's something loose. Or your endmill is cracked.