I noticed, however, that if I resize iTerm to be, say, 1/4 size of my screen (single monitor, 1440p res), it'll work fine but as I increase window size it gets worse and worse. iTerm and Sublime are using hardware acceleration but I can't say to what extent. Enabling/disabling it doesn't make any difference.
I don't even know which part or driver to investigate because GPU seems to be coping fine with heavy tasks but the observed performance degradation when increasing window size somehow suggest it actually is graphics related? I don't know..