As the .jpg format is a compressed picture format, if you use jpg's for model textures, this means that ofp has to spend some time decompressing each jpg into memory.
This can eat up a lot of memory, depending on how many jpg textures you have used on a model, how many of said models are used in the mission and how large each texture is.
Needless to say, it will slow things down a tad, for very little gain.
Also jpg format does not support transparencies I believe.
Better to stick with .paa and pac.
Try PAATool to try and improve the quality of your textures.
I don't know where you can get it, I suggest you try OFP.cz first.
Planck