Hi temp thermoplastic, sounds like THHN insulation...
The newer low-flow Water Pik shower head must be right in the middle of the preferred rate as it cycles a couple of relays pretty regularly. There is some overshoot on the hot side and slow recovery on the cold side, so the water temp is in a state of constant flux. The incoming water this time of year is 42 Deg so that doesn't help. I heated a lot of water and generated a lot of steam with Hot Oilers in the oilfield and out of all the things we did make consistent temp steam was a bitch, constatly on the controls, so I do understand how hrd this is to do on the fly...