Arvados

Workbench is a little sluggish overall, but my main concern is everything between clicking "login" to being able to click something else.

My concern is that it's probably disconcerting / off-putting to naive users for logging in to take a long time. I think people are used to (in webapps) some actions take a while, but usually not logging in. (In contrast, on desktop people are more used to applications "starting up" so to speak (splash screens)).