Or another example - I have 45 and more fps, but there are still visible and annoying frame pauses. Something is wrong in the rendering pipeline.
It would seem you do not know what the rendering pipeline is in the first place, because the rendering pipeline executes the same tasks over and over frame by frame and conceptually can not produce delayed frames.
The OSG model loader can though, texture mapping can, poorly designed aircraft systems can,...
IMHO, FGFS needs multithreaded core, rendering, nasal, weather.
... and you know that because?
FYI, you can multi-thread both the CPU part of rendering (just set the config flag) or Nasal (use thread.newthread() ). However, multi-threading has a synchronization problem - at one point you need to exchange variables in a consistent way, so you need to hold all threads till the slowest thread is ready to synchronize before entering the next step - combined with the actual time to synchronize, in practice that may actually be slower than single-thread computations - which is why I use thread.newthread() sparingly and only for tasks which do not require sync while they compute.
Erik has tested a while ago whether it is possible to run FG with 60 fps steady and no frame delays - he found that on his machine this was quite possible, just not with all options maxed out. I likewise can usually run FG without frame delays (well, short of loading 16k earth hires textures for the first time) at 60 fps on a gaming laptop (last year's release really made significant progress with canvas efficiency).
So usually you're looking at either an overworked hardware or sub-optimally designed aircraft systems - Richard has done some nice tests and optimizations for the F-15 systems as a proof of concept lately.