12.5 inches
?, or 12.5mm thick.
Let's assume 12.5mm thick on possibly a 1.5hp triac, but then 2100rpm is a bit slow for a triac.
With a 10mm 3 flute slot drill you should be able to run around the plate with one cut at about 150 - 200mm/min. Slotting or pocketing will be slower at about a third of open face cutting. I would suggest a two flute slot drill to get the swarf out better, you'll sacrifice a bit of cutting feed speed but benefit with swarf removal.
6082 is your friend with this sort of job, machines well and is cheap to source.