Certain VSTs plugins like the ones from Melda productions (...)
There's rather a lot to choose from there. Can you give a few specific examples?
I've already tried a few of them myself — flanger, ringmod, analyzer, and some other random ones — but I can't manage to replicate any noticeable FPS drops here. (Windows 8 64-bit, running Renoise 3.0.1 32-bit for testing.)
They do all seem to be sharing the same base plugin technology, but who knows if certain ones are behaving slightly different from the others, so it would be great to know exactly which ones you've experienced problems with so far.
It would also be great to learn a bit more about your system in general.
Are you on a laptop or desktop machine?
Do you have an integrated Intel GPU on your motherboard, or a dedicated NVIDIA/ATI GPU, or both?
Single display? Multiple monitors? Any kind of virtual desktop stuff?
In some cases — especially on laptops with both integrated and dedicated GPUs (like my own) — Windows apps will use the integrated graphics by default, while games and other stuff can be configured to use the dedicated GPU through the GPU's control panel. If you're in a similar situation, you may need to explicitly tell your GPU that Renoise should use it. I'm still not sure exactly how that would affect the plugins, though, or how Melda even detects the correct GPU hardware to begin with.
Anyway... If you can mention a few more details — and post your "Log.txt" file from your Renoise preferences folder — maybe it will reveal some other clues.