I have noticed this too, even on other clients, but it does seem to happen more frequently with Columba.
It seems related to path request. If you have heard a node announce since you started the app...
Do you mean from the propagation node or from the nomad node? The reason I ask is because I was pulling nomadnet nodes out of the announced stream and trying to load them. And sometimes, as I said, it would be rather fast. Sometimes it would be slow, and sometimes it wouldn't load at all.