One of the reasons why I think there probably does need to be reworking (rather than just minor optimization) is the fact that an RX 6400 runs the game similar to a Series S despite being on paper somewhat weaker than a Series S. Basically runs the game at low settings with an internal 720p resolution at 30fps. So the PC version isn't that poorly optimized when theoretically worse hardware is achieving the same as the console version (which has additional optimizations.) 

An RX 6400 is about 90% of a Series S in theoretical performance. The Steam Deck is about 45% of an RX 6400 in terms of theoretical performance.

Ostensibly you could make that difference by scaling internal resolution to something like 45% of 720p, but we don't see that. Even at internally 260p the game isn't running at 30fps. Scaling isn't linear on the same architecture here.

So something in the render pipeline is bottlenecking performance on the Steam Deck. RT seems to be the natural answer of what that might be. The Steam Deck's CPU certainly isn't the issue because CPU utilization remains pretty low when playing the game and it has a decent CPU.