Yep, too slow. Need max RPM and increased feedrate.

For a 4mm (on a larger machine), id cut 24K RPM and probably 1000-1500 mm/min, if the machine can cope.

Here I am doing 4000 mm/min with a 6mm endmill (4KW spindle).

https://www.youtube.com/watch?v=QAv-q9P4FjA