Nokia has started rolling out a firmware update for their newest Lumia 1520 smartphone. The update brings the version number to 1028.3562.1402.00xx in EU and 1028.3534.1347.2001 in the US.
Nokia hasn't really gone in-depth about the exact changes this update brings other than stating general fixes and improvements in the change log. This should take care of any minor niggles that may be there but you'll have to wait longer if want to see any major improvements in performance.