My specs are:
Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz 3.40 GHz
RAM 16.0 GB
GeForce GTX 1050 Ti (4 GB)
I tried lowering texture resolution by using dividetexturesize=2 command. Also added the systemmemorycapoff=1 command to avoid potential crashing on load. The game mode is on, the drivers are up to date. I play at 1080 resolution. The game runs off SSD.
I've taken a look at some textures, they seem fine in terms of size, so the issue might be in game meshes, complex shaders or the number of materials used in the scene. I'm no Game Guru expert but it's usually optimized by occlusion culling, shadow culling, terrain chunk culling, object distance culling, delayed shadow rendering, disabling unused objects collision, compressing textures (yours are okay I think), using less complex shaders, etc.. I heard this engine loads the whole game at once without streaming assets which severely affects performance, especially if you have the same model copied over and over throughout the level (the cars, wall modules for example). The less is loaded at once, the better performance, so one of the ways out would be splitting the game into more lightweight levels and merging repeating objects into one mesh or a number of groups which could be culled by distance. The spawns seem to load on trigger only which is good (the number of enemies is quite heavy though, the football team of monsters spawning all at once on each pickup makes everyting lag (I also avoid some monsters to save my ammo, which increases the number of them chasing me around the streets), also breaks the immersion).
All in all, I think you could use profiler like Optick to see what's going on with the game. I've also found this optimization tutorial, might come in handy. I know it's a lot of work for you being an active developer, optimization is crucial, otherwise every project will have someone like me complaining.