Hello
At the time of the backup how is the performance of the Vcenter /ESX server? Has there any recent changes like upgrades to the system (other than upgrading SCV Plugin software). Have you tried vmotioning the Snapcenter VM to a different ESX host to see if that makes any difference?
Have you tried restarting the SCV services from maint login?
Please check for any Name resolution issues.
Edit storage connections in SCV UI and increase the timeout value.
Re-register the plugin using the https://<scv host name or IP>:8144/register.html page.
It is hard to troubleshoot performance related issues over a forum , recommend you open a support ticket to get this addressed