In one of the installation steps, we should advise the user that we're minifying the js and css, and then call getJs/getCss. This prepares the user for the delay, and improves the initial login experience.
We should similarly warn (or add some visual indicator) when activating/deactivating plugins that there might be some delay.
Hmm... how about:
The solution with the Next appearing could cause problem in case the request never comes back somehow.
I would prefer vote for having the user wait for a few seconds when loading Piwik the first time (current behavior). This is usually pretty fast on an online server (6s max in my tests).