The rpm I use is around 12000 (200Hz). If I put the following into that calculator:

6061 T6 aluminum
solid end mill, carbide
6mm 2 flute
DOC1 mm
WOC 6mm
slot/pocket = yes
SFM, FEED 100%
max rpm 12000

I get a feedrate of 900mm/min. I'll keep experimenting with the next parts I cut out to see if I can get a better finish.