Re: Irimi/too slow problem
Standard disclaimer: take in mind that this is online advise. Good fuel for thought, but go to the source your teacher for correction.
Being 6' and 225 myself. I find us big bigger folk tend to turn our upper bodies 1st, ie the shoulders to start the tenkan. The hips should turn the trunk not the other way around. Turning your hips first will help with posture and the quickness of turning. Also it keeps your foot from taking a wide arcing sweep. If you have that wide foot sweep during tenkans, then most likely you have turned with shoulders and not fully engaged your hips. The foot should pass underneath your shoulder span not outside.
I hope that helps