The more mods you have, the more important the load order is. The very best thing you can do after you sort your load order in FOMM, is take the time to read the readme's that come with the mods. Most of them will tell you where to place the mods.
[edit to add] Keep this in mind, if you have 2 mods that change the same thing, the mod that is loaded next will win.
Rule of thumb:
- All .esms should be loaded first, starting with Fallout3.esm
- The last mods to be loaded (.esp files) should be Mart's Mutant Mod files, then the companion files, then any weather mod files and finally your merged patch.
As far as frame rate, if you download and use texture improving files, those will effect your frame rate.