Try to increase Wsuspool memory size and queue length in the IIS setting. Refer to the following steps.|
1> On your WSUS Server, launch the IIS Manager.
2> Open Application Pools.
3> Right click 'WsusPool' and select 'Advanced Settings...'
4> Increase the WsusPool Private Memory limit x4 times, or set to 0 (unlimited).
5> Change 'Queue Length' from the default 1,000 to 25,000.
Then check the results.
Don't forget to maintain WSUS regularly. The basic maintenance job is to run the Server Cleanup Wizard, and you could get more information from the below link.
The complete guide to Microsoft WSUS and Configuration Manager SUP maintenance