Try and have as many teeth as possible engated in the cut. The last diagram above is non-ideal. You want to have the saw just barely clearing the inside of the cut. You want slow then you think, and faster feed than you think. Slow feed means you will be work-hardening the material while cutting it.
Thanks guys, I've updated the toolpath as follows. Should have more engagement.You can control how many teeth are engaged at one time by changing the how far the cutter penetrates your tube. My suggestion for the outside was thinking you had too many teeth engaged at a time and they were loading up with chips. With the 30 tooth cutter you could get hammering if there are not always three teeth cutting at a time.