Editor hangs if changes are made to the material which is currently assigned to a landscape while the landscape editor is open.
Also spams "LogMaterial: 0.03 seconds spent updating 1 materials, 1 interfaces, 0 instances, 1 with static permutations."
Probably also affects older versions, but unconfirmed
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-33708 in the post.