You might also want to check what version bios you're currently running. If it's any version less than 3.7 consider updating to the latest bios. I'm not sure why it should feel any slower if the only thing you upgraded/changed was your cpu. You would have been using the same old hard drive with the same programs running on the i3.
It might also be worth checking the cpu temps with something like realtemp just to make sure the cpu isn't overheating. An i7 does run hotter than an i3, maybe the cooling isn't adequate or the cooler didn't mount properly during the reinstall. High cpu usage as Faux_Grey pointed out doesn't help, likely a background task/process, av scan or update, windows update etc.
The other thing is if the cpu was highly active it's also possible there was high i/o activity on the drive itself as the cpu was accessing data. A background task causing cpu usage and drive i/o activity competing with a current task you're trying to perform whether it's a benchmark, game or anything else will cause a real slow down.