Hi,
How many triangles are in each object ?
As I recall, current gpu’s prefer objects in one call but given a sscenes complexity and the computer its running it , you can start to see severe performance issues .
I"m just staring to have to address similar issues myself so I"m prob. not THe best person to ask, but in general, if you have a lot of onscreen objs’ and as noted are seeing perf. issues, the engine culls the objects when not in view so that should help alot I’d think, but again someone that knows HOW culling works in all sitations its better able to assist you on this particular request.
ONE thing to note about blender, you may not be aware of, and its meant in the best possible way as I admire blender:
Blender ‘edit’ mode has severe lag sometimes for a lot of us , and its not because blender is a poor 3d app, but atm its ‘edit’ mode is in need of a new api given the OPengl code it uses is from very old code. I"m having the same nightmare editing anything on my current mesh because of that, and I"m only using 1.3 mil triangles atm, which is nothing really. Blender edit mode is terrible atm, so its NOT you, just be aware of that. I even have slowdowns on a 66k tri mesh so its not you necessarily.
As long as you have the very latest version of blender, you have the best it can offer , at least UNTIL they fix edit mode and I have no idea when that is coming as likely its a huge overhaul.
For now, can you reduce your mesh triangle count and try again , which would make it a tad easier to work with blender , with the stated caveats.
Also, I"m using a 3d app called ‘Meshmixer’, which has been incredibly invaluable given blenders edit mode problems.
Its free , my autodesk, and I’m positive you would most likely not expeirence the issues you have with blender currently.
Just bing/google for meshmixer and give it a try.
I"ve used it extensively, so if you have any questions just ask here OR PM me or if need be we can ‘skype’ if that would assist you even more , I could take the time no problem.
I have similar slownesss compiling, maybe they fixed it in 4.12 ? I"ve not upgraded yet as my ssd space is limited but I need to. That could fix it, not sure someone using 4.12 would need to adivse.
Something seems very wrong to me. A 40kb fbx file should import fairly instantly on your system.
Your system is faster than mine, and I have little trouble with that small of a file though I"ve never checked my watch against it. My curernt mesh takes awhile to update in ue4, BUT its 1.4mil tri so I expect that 
Hope some of this helps, gl friend