One of the first things I would look into is running a performance recording on the workbook on the Tableau Server, if possible, when it is performant and then when it slows down again. The performance recording will show a lot of information about what is causing the workbook to load and where the load time is being experienced.
There is a great breakdown of how to analyze the performance recording here: Interpret a Performance Recording
Another place to check the "Performance of Views" administrative view on the Tableau Server. This can be used to see if the slow downs of the Viz conicide with potentially just increased user traffic.
I would not say that rebooting the Tableau Server would ever be the first thing I would try. One thing that could be checked is the log folder if it has never cleaned out or emptied. I would suggest a process of archiving and cleaning the logs folder to keep disk space available on the machine running Tableau Server.
Hope these tips get you started!
I can't find "Performance of Views", but do see Stats for Load Times. Same with Performance Recording on the server. I don't have the "Settings" tab at the top, so even though I'm an admin I might not have all admin access. I will check.
For Stats for Load Times, I find the below interesting. For all other dashboards in the workbook, load times are consistent. However, for the dashboard that is performing slowly on the server, there is a big increase mid April. I find this interesting because there were no changes made, no republishing, nothing during this time frame. Last republishing was in March, and there weren't any changes made to this dashboard during that time. This was always the fastest performing dashboard. There are other dashboards that use the same datasource and don't have an issue.