There is a lot to this and it is fantastic. You're level design is great, but some of the environmental pieces have collision, most notably the small strands of grass in the wild west, I'd often get stuck on them or they stop me moving as I bump into them. turning off mesh collision in the panel will do the trick for this.
The main hub level tanks my PC quite a bit, I reckon this is an Unreal problem, myself and my students have had countless problems with 5-5.5 in terms of optimization. Where the later versions of 4 (most notably 4.27) had a lot less. Other than this Downsized is a fantastic base to build on, excellent work. :D