Frame rates with eGPUs are an interesting beast. You can be CPU bound, GPU bound, or bandwidth bound by Thunderbolt 3.
And even if your CPU isn't being utilized 100%, that doesn't necessarily mean that you're not CPU bound due to it being thermal throttled (my Mac Mini's CPU runs @ 90 C and above when gaming) or the game being poorly optimized for multi-core processors.
I play Rise of Tomb Raider and Division 2 in 2K resolution at near 60 FPS. I can play GTA V in 4K resolution at 60 FPS. But it doesn't matter what I do in Monster Hunter World, I cannot get consistent 60 FPS. I've even gone so far as to reduce the resolution to 640x480, but it doesn't have ANY effect on my frame rate. Those who play on a less powerful system than me with an internal PCIe GPU obtain consistent 60FPS in this game.
I asked a very similar question on eGPU.io and received a very thorough response. You may want to check it out here. It's the post near the end of the thread by a member named P-Mac.
https://egpu.io/forums/mac-setup/pcie-slot-dgpu-vs-thunderbolt-3-egpu-internal-display-test/paged/3/
He writes, "Display resolution (aka rendering resolution, for games with built-in scaling) is independent of texture resolution". I think that this is what's going on in my case with Monster Hunter World.
In addition to all that money, it's yet another price we pay for this compromise.