One of my colleagues hit this last week:

TF401190: The local workspace XXXXXX;XXXXXX has 112202 items in it,
which exceeds the recommended limit of 100000 items. To improve
performance, either reduce the number of items in the workspace,
or convert the workspace to a server workspace.

Fortunately he had a few older release branches he could remove. I didn’t know there was a recommended limit. I found this overview of local workspaces:

Local workspaces have scalability limitations due to their use of the local workspace scanner which checks for edited items. Local workspaces are recommended for most of our customers, because most workspaces fit into the “small” or “medium” category in our view – that is, they have fewer than 50,000 files and folders. If your workspace has more than 50,000 items, you may experience performance problems or TF400030 errors as operations exceed 45 seconds in duration. In this case, splitting your workspace up into multiple smaller workspaces (perhaps one workspace per branch), or switching to server workspaces is recommended.

Server workspaces are optimized for performance at large scale and can support workspaces with up to 10,000,000 items (provided your SQL Server hardware is sufficient).

It seems performance for local workspaces has been improved, or maybe the limit has been adjusted due to average hardware upgrades. I can tell you though, our SQL Server hardware is not sufficient, greatly due to TFS using too much disk space.