WSUS on Server 2019 is not version 3 - it's version 10.x
WSUS on Server 2022 is also 10.x
WSUS on Server 2016 is also 10.x
WSUS on Server 2012 is 6.2
WSUS on Server 2012R2 is 6.3
Are you performing the proper WSUS maintenance including but not limited to running the Server Cleanup Wizard (SCW), declining superseded updates, running the SQL Indexing script, etc.?
https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-8-wsus-server-maintenance/
To quote myself:
Just because you’ve installed a new WSUS server, doesn’t mean that it’s clean or optimized; it just means that it’s NEW!
Also, your downstreams and upstream also work depending on how you have your systems added into each of these. Having 4 downstreams with only 1 of them housing 600, 2 of them housing 400 each, and the last one housing 500 as the upstream - this would give you a general equal usage. But if you had 800 on the 1 downstream and 2 of the others are only handling 200 each, and the upstream is handling 300, then you're taxing the 1 downstream.
That being said, a single WSUS, when sized appropriately, can handle 25,000+ clients so you should be having no issues.