We have vRA 7.2 environment having one vRA appliance,External vRO and One IAAS Windows server, when we open any item or open Catalog item it is taking time to load the form.
Is there any suggestions or best practices for performance tuning for these components ??
You shouldn't start with any performance tuning until you've identified if that will actually solve your issue. From your description, that's probably not going to do anything. There are lots of factors at play in vRA, and you've stated very little about your environment, how much resources you've allocated to each piece, what this catalog item is, etc. Also, do keep in mind you've deployed a minimal vRA instance. This is not meant for grand scale and size and usually is only recommended for PoCs and pilots.