Can confirm, 7 is the worst one. I'm doing optimal or close to optimal routes on every stage except 6 and 7. 6 has a global timer and some tricky jumps on the flames, but apart from that is manageable.
7 has a global timer on the spike pistons, which can only be adjusted by stopping or landing on the mine carts to push yourself forwards a bit. There is a pause glitch to stop one of the spike cycles from killing you, and the boss at the end has really bad RNG. It is pretty bad all around though with RNG, every stage has parts that can kill you even on a perfect run.
3 has a pixel perfect early jump on the flame bridge, then a split second shot to shoot the B pickup. I don't want to do it, but it saves around 8-10 seconds so I'll have to learn it.
Nice, pretty good run. I think Level 7 would be an absolute bitch to speedrun well.