Why does creating interface take such a long time?
The new pfSense 2.0x is a bit slower when it comes to GUI functionality VS version 1.2.3. That's probably all right given the new features added (but still not a good thing). However, adding interfaces take a really really really long time. There should be a way around this to make creating interfaces, enabling them, etc…much faster.
USB Interfaces are EVEN slower.
I haven't noticed any slowdown in adding interfaces, but then, even though I have used many different pfSense builds it is months since I added a new interface.
What sort of interfaces are you adding? Maybe the slowdown is specific to (say) PPP interfaces. How many? Maybe creating a new VLAN gets noticeably slower when you have a few hundred. How frequently? Maybe you don't really need to be adding new interfaces every day. Maybe there is a more effective way to do what you want to do.
What system are you running on? Maybe if you have a few busy VPNs on an Alix system the GUI gets slow because you really should have a faster CPU (and maybe the packages need more memory).
Actually in many configurations, pfSense 2.0.x is much faster than 1.2.3 with interfaces. Especially with large numbers of VLANs.
There are quite a few factors that can go into it though. It really depends on the specific action you're taking that you believe feels slow.