Holy Cow that's a lot of AD groups to be syncing with the Tableau Server! I only add them when requested but we do have Guest Access turned on in a few of the Sites.
According to my Server viz, http://myServer.com/#/server/analysis/BackgroundTasksforNonExtracts:
For 38 AD groups to sync on my Server it takes less than a minute to do all of them. The actual time to sync is from 0 to10 seconds with most of them being 1 second. They all ran on the same backgrounder process. My sync runs every two hours.
yeah, we currently have about 300 groups that sync nightly on our production server. The sync times are same as yours, but 1 second times 30,000 groups is 8.33 hours. Most of them are added by site admins in our 30 sites, but we are starting to consolidate most of the sites into one. That means the task will fall to my group.
We have 4 backgrounders and each night when they sync they all go to one of the 4. When I manually synced they took 17 minutes, but when they were synced automatically at 11pm it only took 3 minutes. I will see what happens after a few days on the dev server with 1000 groups, and then try adding more.
If it ends up taking a long time then submit a case to Tableau Support. If they can't offer a solution at least it will be logged as an issue that the management team will see