For a while I have been trying to troubleshoot one clients slow Sharepoint performance.
This client originally had problems with the Onedrive For Business Sync Client with document libraries failing to sync. Microsoft advised not using sync but mapping to Sharepoint in Windows Explorer.
This resolved one problem, however when navigating the folder structure, they would get slow performance in refreshing the folders and some timeouts.
They also had issues when saving document. These documents would appear as offline copy.
After some more research I found this link.
Once installed and run, it picked up DNS resolution time to be more than 25ms.
Looking at the DNS settings, they were using Googles open dns 22.214.171.124 and 126.96.36.199.
I changed them back to the local ISP dns server and thinks have got a lot better.
Anyone having issue, try this method and let us know how you get on.