Try to reset your network settings?
Settings > General > Reset > Reset Network Settings
Mine is the same way. App Store servers are pretty slow. I have really quick internet, and a nice router, but it takes ages to download apps.
Ok I found out download speed fluctuates a lot when downloading apps from the Apple App Store.
Like 100KB/sec to 2MB/sec then 6 MB/sec then back down to 16 KB/sec, etc. Is this Apple's App Store servers or my internet connection ?
Try turning on VPN (if you subscribed to such a service) and see if it makes a difference.
I once had a similar problem for a period of time where my iOS apps simply wouldn't download over wifi at night. VPN was a temporary fix until the problem went a way a month later.