Hello,
I also have this problem. I found a temporary workaround, which help. Not a solution at this problem, but a way to have a decent fps with a good rendering with SMP.
My configuration is X-plane 10.45, NOAA weather, RWC, SMP 3.2, RTH, flywithlua, and minimum plugin (planes, airport navigator, 124thATC).
When I had this issue, everything run smooth (20-30 fps) and after a flight time, about tens of minutes, i had a fps drop, about 6-15 fps. I seem to remember the issue started when I installed RWC. i had no issue with SMP alone. I started to think my rig was overloaded (old but overclocked, see below). When I see that other people have the same with a strong rig, I start to looking for a software solution. I notice that when I disable SMP everything run smooth with default clouds.
When I used SMP alone, I had only the tree layers of clouds of x-plane, i used to generate manually a global weather.
With NOAA there is more layers, and the weather is much more complicated (wind layers, rain, etc), and nonstop reload. there is a lot of parameters managed by the simu.
A thing makes me think that it's a graphic overload somewhere. I have to reduce drastically the graphic options of SMP to have a descent FPS during the whole flight. (area 4096). It's not realistic to have a group of clouds witch follow the plane ! But it's fast.
When I run with default clouds or clouds HD, it run fast too. A solution is have both !
The only workaround I had found to have the great environnement of SMP AND a really good visual of the world around AND a good performance is to run SMP together with default clouds of x-plane.
It may be a temporary solution, until the issue is resolved.
Then you have a benefits of SMP directy around the plane, and lots of clouds until horizon. Both of the clouds are mixed, and you have a nice rendering.
For this i use the lua script "Default_cloud_enhancer.lua". To enable the cloud mix, I add the line set( "sim/operation/override/override_clouds", 0) in the script (open with notepad)
For cover all the sky I use Clouds HD v2.0 (free texture remplacement), but you can use default clouds.
You must have RTH and flywithlua to do this.
This workaround works only with version 3.2.0
For now I stay with version 3.2 with this config, until the reason of this issue will be identified.
The only disaventage I found, when you fly at dusk, you have black and white clouds ! and you see the limit area of SMP.
My SMP settings are :
cloud area covered : 4096
Puff rotation : 0.1
Cloud style : crisp
god rays : OFF
cirrus : med, texture 4
reflexion in water : OFF
sparse particles, lens flare : OFF, cloud shadow : 0.6, cloud/Terrain : 500
My graphic options are :
textures compressed, very high. fullscreen 1680*1050*32, fps unlocked.
trees : invaded, objects and road : tons, cars : winter. objets distance : high. airport details : default, shadows : global (high), reflexion : complete. only one box checked : "the runways..."
in the special effect box : all checked, 2x SSAA+FXAA. FA : 16x.
Clouds details : 10%
Rig : Core2 E8600 overclocked at 4.5 GHz, 6GB RAM PC8500, SSD, GTX 970 4GB.
I have 2 AI planes "XPFW" (lightened)
I salute the work of Sundog for this great plugin I use since version 2.0. I know that it's a really hard job to found an erratic bug like that. I hope you'll identify soon the solution of this issue, which seems to be hard to found. There is a lot of interactions parameters stake for a while (new xplane updates, new sofware RWC with interactions with many weather infos plugin, etc) and can come from anywhere. Hoping find again a total cover area with last version of SMP. Good luck.
I'm sorry for my English, i'm from LFOT ! :-)
have a good day!