My iphone 6 cant syncing to my itunes and it make my computer so slowly

I have a iPhone 6 IOS 8.4 (12H143) 64 GB before syncing it to my computer without any problem but from a month I couldn't, sometimes I plug it to my computer for more than 3 Hours and after that I have a message that cant syncing and during that process my computer became so slowly I don't know why please advise