Don’t worry about that warning, it’s unrelated and has actually been removed in 4.6 since it fired a lot when things were actually fine.
I’ve fixed your perf. issues when logging a lot of output log text: https://github.com/EpicGames/UnrealEngine/commit/7775242a9362fa07cfbba8b8270bf2f946852c25
It turned out to be quite simple. FTextLayout::AddLine was causing a re-flow of the entire document when a new line was added, rather than only flowing the line that was being added.
I’ve not being able to reproduce your hang when left-clicking (or right-clicking) in the output log. Does that happen all the time, or only when there is a lot of text in the output log?