You should just wipe your phone start over. Or go to a Genius Bar for support / call apple. There is no possible way for someone to randomly “hack” your iPhone or slow down the network speed probably just a bunch of junk subscriptions in calendar and safari notifications for websites where the adds are coming from and other things just clogging up your phone.

I say start fresh / wipe & new apple account. or take it to someone technical to have a look at your phone.