Note 2: slowdown doesn't mess up gameplay, at least with Cheat Engine. Den might be thinking about Hourglass, which is notorious for being glitchy.
Not quite true, although I don't think it actually messes with the jumping in any way odds things tend to happen when using slowdown, for example if you activate a save in slowdown (not sure if a certain frame rate is required for this) and activate it again before the save switched back to it's original sprite the 2nd time of activation will not work for some reason.
Well, I have to say that I've never tested the macro with slowdowns. So that's not what it makes the macro inconsistent.