Windows 8's new 'don't-call-them-Metro' apps use a process called Runtime Broker to manage permissions. Under normal circumstances it should use only a few megabytes of data. However, in some cases, a memory leak may cause it to balloon up to a gigabyte or more.
The issue involves how the process handles live tiles. Under certain circumstances a request for an update to the tile may set aside an amount of RAM that never gets released. Over time, that allotment grows and grows until your system grinds to a halt. Jeffrey Keys, a developer of an app once afflicted by this problem, elaborates:
The bug manifests itself when calling TileUpdater.GetScheduledTileNotifications. The leak may not be obvious unless you have many tiles scheduled. In my experience it happens regardless of the number of scheduled tiles, it's just more apparent when you have more.
The issue has been reported to Microsoft, but there's no confirmation of a fix yet. In the meantime, if you find Runtime Broker is using a suspiciously high amount of RAM, try uninstalling any Windows 8 apps you have that utilise live tiles, one at a time.