Is there a way in Piwik that I can check how long certain pages are loading for mobiles, either on average or by individual site visits?
If not I would settle for a way to see average load times, if that's possible at all.
The only way to achieve this in Piwik is to have your mobile pages served from a dub domain in which case you will be able to track the average load times based on the sub domain. Otherwise if you have specific page URL's for mobile pages you can check the breakdown of page load times by page at
Actions > Pages and Actions > Page Titles.