I’ve noticed the rendering speed has been way slower in Unreal 5.3 compared to older versions.
I tested by converting an older scene and compared the rendering speed in 5.2 and 5.3. 5.2 renders it in 2 seconds and in 5.3 it took 16+ seconds. Both were rendered using the same movie que settings on the same computer.
What could be the cause to this problem and is there any fix?
Oh, I have the same problem! 
When I converted my project from 5.1 to 5.2, no problem but when I convert to 5.3 I got massive gain in my render times.
I started to think about the updated screen percentage system in version 5.3, and I conducted some tests over the weekend. When I render my project with a screen percentage of 83% or below, I achieve good render speeds, maxing out all of my CPU cores. However, if I increase the percentage from 84% and up to 100%, only two of my CPU cores reach maximum usage.
I also conducted a picture comparison between 100% screen percentage in version 5.2 and 83% in version 5.3. The results showed that 5.2 with 100% produced a slightly better result.
I ran into the same issue - using UE for Archviz mostly, the fast path tracing was a reason for me to use Unreal in the first place.
It being so slow suddenly makes my question if this was the right choice… Have you managed to get rid of longer render times by any chance?
Thanks!