I also discovered something very interesting. Calling textureSamples() in GLSL reveals that a 1x MSAA texture actually stores two samples.
That means the engine is currently doing a lot of extra works because 2x MSAA is always enabled, or at least some aspects of it are, at the very least extra post-processing expense and memory bandwidth. So if I use rectangle textures or something, instead of 1x MSAA, it should be faster when MSAA is disabled.