seth, if this problem occurs, try to deactivate RWC first and SMP second and have a look what happens. After that, activate again RWC first and SMP second. Perhaps you have to set the weather options again in XP. I expect your fps performance is back again after this.
The solution which helped tolix does not work for me, unfortunately.
It looks like a kind of buffering is filled after some time and is never cleared. I hope one of the dev's has this on his list already.
EDIT: Deleted filepage.sys to ensure XP is using the whole physical memory without paging and I hoped it would improve the performance a bit, but the sim was running out of memory (even with 16GB!).
I went back and working again with pagefile (on SSD). I don't use W2XP any longer, that gives me fps 12-30 with setting objects to "tons" , distance "low" , "extended DSF" . I still use HD-mesh. Sometimes I have even higher rates, depending on the plane (new SSG E170 Evolution is consuming much fps in flightdeck view). I always telling about flying in big city areas with busy airports and medium to high settings. So my conclusion: Skymaxx is not the mainly reason for fps drop(it is also) - rather the scenery and amount of objects are (HD_mesh, oms).
by the way:_ flying over water(sea) in clear sky pushes fps going through the ceyling - and now set weather so SMP has to draw many clouds and compare the fps you get what you have over land. You lose some rates of course - this is what SMP is consuming. And water is also a texture in XP but without objects !