When trying to add widgets, it seems you are forced to "wait" for the widget "preview" to load before clicking the widget to add... otherwise your clicks simply get ignored.
This is terrible, especially when your server is slow to process the data.
It used to work like this, this was changed during the redesign.
Your Piwik should always load fast. If it doesn't, please increase the time out in the Admin > General Settings, for example to make your reports refresh only every hour.