Pre-filter really a good idea?

Sooooooo, I removed the snorkel and fitted an oiled uni-filter as a pre-filter ..................... seems a good idea to save the main, paper filter.

Went away for the weekend with the new filter in. Lotsa dirt/mud/rocks and tar.

But the bike didn't seem to go as well. It's hard to put a finger on it but opening up from low revs (3rd/4th gear) it seems to hesitate/bog for a while before hitting higher revs. I'm used to having instant wheelspin when I want it or instant/hard acceleration when I want it. It just seemed a bit pap - not it's usual self.

I don't think anyone else would notice, but it's my bike and I can feel something ....................