Keep the chain sharp, and I mean sharp, if the mill is NOT throwing chips, change your chain or stop and sharpen it.
The saw has to be set up right to run at full chat under extreme load for an extended period of time, most of the time a saw is run to cut cross grain even on a large log it will only run a minute or two, with ripping a long log it might run 5 to 10 minutes at full chat. The saw should burble slightly when in the log and running full out. If you need it, set up an aux oiler for the chain, this can be as simple as a steel can with a screw on lid, mount it over the chain on the out board end, punch a small hole in the bottom of the can, if the oil does not drip out enough loosen the lid.
Make darn sure that the log is set up so the travel of the mill is downhill, this makes a HUGE difference in how much work it is, if it is level, or God forbid uphill, you have a lot of hard pushing to do, if it is slightly downhill, the mill will just about pull itself through the log.
On longer logs, plastic felling wedges are your friend, use them, the saw will work a lot less if it does not have to fight the pinching of the kerf behind the bar and the wood will come out with a much smoother surface.
I cut my slabs all to be around 7cm thick, I think if I were to ever do it again, I would have gone for 8 or even 9 cm thick, takes longer to dry but I can then rip that into two boards with my bandsaw down the road, making less work on the mill.
A couple more things bring a shovel and a rake, as you go you will spend time on your knees working the mill, you want a nice smooth area to work!
Cheers!