Hi Dan,
In general, a ProGet feed will be slower than a "real" Debian repository. The reason is that Debian repositories are just static file system like this:
http://ftp.us.debian.org/debian/dists/bookworm/
There is obviously a lot more overhead with each ProGet request, since index files are dynamically generated, involve connectors, need to be permission-checked, etc.
In addition, ProGet indexes the Debian indexes on-demand, which means downloading all of the Contents-*.gz files and indexing those. Like in here: http://ftp.us.debian.org/debian/dists/bookworm/main/
While these files are cached, they do need to be updated when the remote repository updates.
And note that each web node maintains its own local index cache, so you will see these long-running requests multiple times. Some organizations will periodically "warm up" the indexes periodically by just hitting the InRelease endpoint.
Hope that helps,
Alana