I do not not experience an fps loss over time, in either P3Dv4.5HF2 or P3Dv5.
You are running 3 displays...so I think Gerard's setup may be of a lot more use to you than mine. I run 4k @ 30hz (or sometimes 50hz or 60hz) but only on one display, with a 1080ti. You have an entirely different set of performance challenges with you running 3 displays.
I run an 8700K at either 4.8 or 5.0 (this time of year I usually turn it down to 4.8 as my h100 cooler is a bit light for what I ask of it), with hyperthreading enabled. In v4.5HF2 I am running an affinitymask, where core1 is excluded (i.e. 11 11 11 11 11 01). This frees up Core1 which allows its parent core0 to run full blast. I have changed that over time from various other AM's but presently I'm getting really good results with the above. I also run my add-ons with the FFC mask (that is hexadecimal) which puts them on later cores (in other words, keeps add-ons OFF of Core0 and Core1. I do that via a batch file, which if you'd like I can post. I run the batch file first, which is a series of "start..." commands and runs Pilot2ATC, UTLive, LittleNavConnect, ActiveSky/ASCA, and SimSounds. Then I run P3D.
In the sim (4.5HF2) I have three settings profiles saved. All of them use Unlimited with vsync and TB ON. This I found gives me the smoothest, most stutter-free flight. My most luxurious profile ("General Settings Profile") has most sliders right and many shadows on except for Autogen Radius (medium) and Autogen Buildings (a known fps detriment). My most draconian profile ("London Night"), which I rarely have to use, disables HD lighting, all shadows, and has AG buildings down even further...this is a profile I might use at Heathrow at night, where shadows and visuals don't matter so much, and I'm able to maintain my framelock. Vsync is effectively a framelock on my system. My other profile is "London", which implies London Day, and allows me to hit my framelock in all conditions in the worst location in the sim (Heathrow with weather and AI and complex addon airport and aircraft). If your fps can hit your framelock (vsync) that is a big step toward smooth flight.
In P3Dv5 I have the same three settings profiles. However, with v5 performing better, I have more headroom for visuals. With that in mind, I am able to refresh my monitor at 50hz, or 60hz (although I typically still refresh at 30hz, for complex airliner ops), and maintain that fps (50 or 60) in rural areas. This means my "General Settings Profile" is largely unchanged (it is still Unlimited with vsync and TB ON) but it allows me to get 50 or 60 frames totally smooth. It's really nice. And of course it is noticeable. i've been here 20 years and I cannot recall anyone in the AVSIM subforum saying you can't see/notice over 30 frames, I don't know where that came from. Maybe some other forum. 40-50-60 is way better than 30 *if* it can be smooth. But smoothness is key. I'd rather have 30 smooth than 40 stuttering, and I think most simmers are on the same page in that regard. With v5, it is even better and doors are opening for us to have 40-50 or 60 smooth, at least with rural GA flying, and I'm still experimenting toward that end with airliners and urban areas. With you running 3 screens I would not set my bar that high though -- first take small steps and aim for 30 smooth on all three, if that's possible.
If I were you I would temporarily reduce my setup to just one screen. Get that one display smooth and get your setup right for that. Only then would I add a screen. I would also temporarily run no add-ons, establish a baseline, then start adding add-ons one at a time and test for smoothness. Again, Gerard's setup should be very helpful when you start adding displays.
In v5 I am (right now) running 2048 textures with Orbx Global, OpenLC Europe, and OpenLC North America. I have never done specific testing with vanilla textures vs. Orbx and load times or stutters, but I would think that your texture resolution would have more of an impact on that than anything else.
My other hardware are Samsung M.2 drives (two of them, for OS and v4.5HF2), and I actually recently bought a straight SSD (1TB) which is presently holding P3Dv5. I was running out of room on the M.2's. SSD is plenty fast enough file access for us. As far as mem goes, I run 32GB of 3200 low latency (I'm not at home to check but it was something like CL 14 or 15ns if memory serves). CAS latency matters when building a sim machine.
Anyway, hope that's not too long. I hope you find a smoothness nirvana.