This is normal, actually. If I recall, it is related to the favicon not being available. The ULS logs should have more information on which file is unavailable.
Slow page load could be indicative of multiple issues, such as poor overall server performance, be it SharePoint or SQL (network, disk, CPU, memory). Or it could be poor page structure (i.e. adding a bunch of List webparts on a page that make expensive queries).
If you haven't, also take a look at https://learn.microsoft.com/sharepoint/administration/cache-settings-operations.