well, whatsup is used for transfering a lot of media files. Delete them. Or some of them from time to time and that's it. Internal storage has nothing to do with how slow the device is. It's mostly because of android itself and OEM bloatware plus users (cr)apps that are just piling up with 86358962587 background services running 24/7. My Z2 tablet is close to one year without restart and still run as fast as after restart. With official MM FW, just rooted. uneducated users are the problem, not that much android as such.

EDIT: to be precise, 303 days without restart.