Experienced the same issue, please have a look at Privacy->Analytics. If you (like me) find a lot of errors related to mediaserverd, then 'reset network setting

Experienced the same issue, please have a look at Privacy->Analytics. If you (like me) find a lot of errors related to mediaserverd, then 'reset network settings' will likely fix it (like it fixed the lag on my 6S after upgrading to iOS 11).