Can you try running the following from your vCenter servers:
"%VMWARE_PYTHON_BIN%" "%VMWARE_CIS_HOME%VMware Identity Serviceslstoolscriptslstool.py" list --url https://localhost/lookupservice/sdk --no-check-cert --ep-type com.vmware.cis.cs.identity.sso 2> NULL
or this if you are running the VCSA:
/usr/lib/vmidentity/tools/scripts/lstool.py list --url http://localhost:7080/lookupservice/sdk --no-check-cert --ep-type com.vmware.cis.cs.identity.sso > /tmp/vc_identity.txt
We had a very similar issue with the vSphere Web client being extremely slow to login and querying inventory data was a nightmare. Turned out that some how through the upgrade process we had ended up with two service registrations (with the same cert). Removed the bad one and magically everything started going “fast” again.
This only showed after we had installed SRM on the vCenter servers. Before that it was slow to login but workable.
Anywho – see if you have double entries and then we can go from there.