Jazz is right about MDF ..to slow and it burns.. to fast and chips or can jam ..plus u need to balance spindle speed with feed rate, tool radius increases or decreases contact speed, I found 2 flute good but I heard 1 flue maybe better.
The way I worked out the best feed/spindle rate/speed I set up 20 8 inch parallel lines to 10% over min feed rate changing each one until you have the last one at 10% below max feed rate. Using a 2 flute 6mm end mill I set the cut depth to 50% of the diameter.. 3mm and ran the g-code. Spindle speed set at 50% max. That should give you a good indication of how well your machine copes with the MDF. If the say the middle 4 or 5 lines show no burning and u are getting a fine dust then you are heading for the right combination.. experiment.. increase spindle speeds until you reach a good balance between feed rate and cutting.
BTW I cant emphasis how important dust extraction on MDF is, both for you health and cutter life.