There are various possible reasons ... you could raise it with Tableau Support and maybe they can go through your logs. There are some questions you might want to check:
1. Do you have anything else running on your Tableau server box? any other processes that might be running during morning, Go through your logs and check any memory/cpu spikes etc (what is the time range, whole morning?)? Do you have more slaves?
2. Network bandwidth, can you test if you have decreased performance on your network during that time and how is the Tableau Server/ Tab Desktop communicating with the database / can you check the users's connectivity to the server, are there any VIPs, proxy issues etc
3. Is this happening on Tableau desktop or only server?
4. You can also try to use the Tableau Desktop Performance Analyser and see if it's giving you a better insight
5. Do you have any re-occuring Tableau reboots?
6. How is your caching configured on your server (more or less frequent)?
In support Case Number 04539540 so far of the help addressed is not present.
1. On the server except Tableau there is nothing. It is created especially for this test.
2. We check reports postorenny for extraxt to iskolyuchit network problems.
3. Occurs on Tableau Sever 2018.1.6 After several hours of loading of TabJolt
4. Noticed that in the morning on a tomzha the report, Bilding View is increased
5. We do not overload the server. As reset helps, but it is not an exit for industrial service
6. Refresh less often
With switched off by shedulers there is a record on a disk in the C:\ProgramData\Tableau\Tableau Server\data\tabsvc\temp folders. Maybe is not enough what resource. For example now random access memory it is free 77Gb from 512Gb