I "beat" isaac on the Switch (one Dead God), then got it all again for the Steam Deck so I could have mods. Unsure if the mods themselves are tanking the performance, but I don't really care. It's up there as one of my favorite games. The creativity of the mod creators has kept me coming back to play custom characters and challenges.
The modding community really is insane. There are a couple of fan-made expansions like Fiend Folio that massively overhaul everything. I hope this update tries not to break mod support.
Oh yeah. Got that installed. Was fun because I hadn't worked with Linux systems before, so finding the proper file locations was a little challenging for myself. Custom characters have post-its and I don't get the big banner saying it was installed wrong. Even messed with performance settings the deck offers for each game and lowered those since, well, Isaac isn't exactly the pinnacle of graphics. Even now, just custom characters, alt paths and their additional effects (rain drops, ash, etc) tanks my performance. Unless there's some magic that I'm not seeing, I've tried what I know. It is a little annoying when games that have actual models, and a lot of them at once, don't have issues at all (Deep Rock Galactic Survivor being a prime example).