Hi there and welcome to Panda3D.
Regarding the GUI toolkits available in the engine, the Rocket library has been dropped since it wasn’t supported anymore. But, if you’re looking for a simpler way to create DirectGUI UIs, I’d recommend take a look at the directGUI designer I’ve written a while ago: DirectGui Designer
In the forum you will also find other useful things that’ll help creating UIs.
As for the performance part, is there anything specific that you have encountered? In the past I’ve been able to build quite large UIs without that much of a performance drain or lag.