Do orthotics for underpronators impact performance ?

Something this serious I would think it better to consult a sports medicine doctor versus an internet forum. 😕 Too much chance of Joe Schmoe telling you the wrong thing and making your situation worse.

I'm not going to give advice as I agree that professional advice should be used for your particular circumstances, not a forum. I can only give my experience in case it provides any info on how others deal with it.

I wear orthotics (extremely flat footed, and one leg slightly shorter than the other by a few millimetres) all the time and my podiatrist said I should wear them when cycling although he didn't seem to concerned either way. I'm due for minor knee surgery soon and my surgeon has also said the same. I think my problem occurs more from foot impact when walking/running so I don't think it'll make much difference in impact when cycling but I've been wearing them for years so feel uncomfortable without them. One orthotic is slightly thicker than the other to account for my differing leg length and I do think this may make a difference to reducing knee pain as otherwise I could be unbalanced.

As for other aspects of performance - I don't know, but if you find anything out I'd be interested to hear.

"And what about back's heath ?"

Not sure what this means.

Again I would suggest professional advice over forum hearsay.

My personal experience is that I got orthotics when I couldn't shake off shin splints when running distance. They sorted that out fine. Most times I ride I don't wear them but don't forget to re-adjust your saddle height if you do decide to wear them on the bike. Personally I find little difference between wearing them or not when I ride but a huge difference when walking.