I've been playing a bit on Xbox cloud on my steam deck and unless I'm losing my mind it's been a far superior experience compared to the Switch port. Based on the latest comments from R&B, I don't think a significant performance update is coming anytime soon for Nintendo, if at all. I hope they prove me wrong!
I mean they originally when the KS first was announced they were targeting the Next Gen Nintendo device rather than the Switch. The PC version was the source development platform with ports for rest.
While a better performing Switch port would have been ideal, I guess this was one of those case despite the game being 2D Sprite based, it's doing a bit more mechanically under the hood that is giving the Switch issues hit an adequate performance target. Same issues seemed to arise with the Bloodstained: RoTN port too which much like Eiyuden was a late addition to include the Switch.
Although considering stuff like Octopath Traveller II and Unicorn Overlord existing and having decent Switch performance, I don't see why their can't be some performance improvements for Eiyuden. It may just come down to the fact that Unity isn't that easy to optimize compared to Unreal Engine projects. On Switch at least.