Philipp, is this a production server? How much load is on this machine (how many dashboards/data sources/users)?
This is underpowered for a production machine. 32GB for Windows Server plus other things PLUS Tableau is not going to be a fast machine. Tableau really wants to be the only application on the server.
And that i7 processor is definitely going to be slow in that configuration.
Can you tell from the boot logs what's taking so long?
Have a look at the tabadmin.log to see why it's taking so long. We recently upgraded to 10.5 and saw some slow down when doing a tabadmin restart, it used to take about 5 minutes prior to the upgrade, now it takes about 15 minutes, it wasn't enough of a reason for us to hold up the upgrade, but when we looked in the logs, it appeared that some events were being done multiple times (i.e. checking of licenses).
Also, take point to MG's comments, Tableau should be the only thing running on this box.
Jeff, poke around for information on cleaning up old license versions. I forgot about that issue: the change in license format that was introduced with 2018.1 can cause some issues if old/trial license information is hanging around.
I've seen this be an issue with Desktop, but it might be the case with Server as well...