Having more files be loaded after the game increases the performance hit.I have arround 30 small mods(none of them conflict each other) and if these mods were to be put into a single file like the DLCs(say Dawnguard) would the performance hit be decreased?
It's the size and effect of the files that decreases your performance. Putting them into one file wouldn't help at all.
Although it's usually possible, I can't think why someone would do it. (unless they'd hit the file number limit)