tl;dr Only optimize it enough to run on newer hardware.
I mean, from my experience playing on an old iPad mini it’s going to be really hard for you to support those older devices. Literally 90% of the games I played on it gave me really low framerate and most even crashed at some point. The only games that worked were (not surpisingly) really low-detailed old games that were made something like 10 years ago and maybe a few obbies. There’s a reason that most of the popular games on mobile are such low quality, being that they’re all a lot of players can really enjoy. There’s probably plenty of kids using ancient hardware, in fact didn’t Roblox say most of their mobile users are using iPhone 5s or something?
Luckily Roblox is working on making rendering parts cheaper so you might want to try optimizing your place to work with that. Also, I think some people believe that using only smooth plastic as a material for parts will help reduce lag (At least the mobile version of Work At a Pizza Place does), so you can try that too.
But seriously, I might be missing a huge market, but I’ve stopped trying to support the older devices. It takes the fun out of developing anything that’s not the absolute most basic (visually) and just makes things so much more stressful. I’d optimize it as much as you can, for sure, but if your grandma’s 10-year-old iPhone can’t run it don’t sweat it.