To share my experience : I don't have the biggest collection out there (~3k), but Cover handles them fine.
I tried both on local drive + network (my wifi being very slow).
On the local drive it was fast to add (less than a minute i'd say), and quite fast to refresh (the thing that spins in the background, when you start the app : same time approximately). The navigation inside the app is fluid after that and there is no loading between folders.
On the network, it was a bit slow to add or refresh (several minutes), but then browsing and opening was fluid.
It seems it's just the adding/refreshing time that may be long with a lot of comics, after that the app responds fast.