Ever since it was released, the vSphere Web Client is not the most responsive of web GUIs. After some digging, you can work around it to boost the responsiveness! It looks like the Java-based front-end is a bit starved for memory under the covers. Now, YMMV and it might put you in an unsupported state, but tinker if you wish.

On your vCenter server, edit the wrapper.conf file at:

c:\program files\vmware\infrastructure\vspherewebclient\server\bin\service\conf

Change:

wrapper.java.maxmemory=1024m

to

wrapper.java.maxmemory=3000m