The flow of hot water is detemined by the pressure at heater's outlet. And pressure at outlet is detemine by pressure at inlet. In other words, hot water is pushed out at about the same rate as cold water is pushed in. Toilet fill valve is a full-bore thing, so when tank is refilling, it can drop the pressure throughout the system by quite a bit. In an ~80psi setup, this drop is not enough to reduce flow to other fixtures, but in your case it is. Moving to PEX is not going to cure it.
What you need is pressure booster