Comment 1 for bug 1312284

Same issue here but with Nvidia.

Ubuntu 14.04, Nvidia GTX 780m.
Tried nouveau, but doesn't yet support my card properly so I can't say whether or not the resize lag is present with the OS driver.
Nvidia proprietary drivers from Ubuntu repos, as well as the most current driver from Nvidia all have the same issue.

Resize - Normal = laggy
All other modes are fast. All other screen re-draws are fast.