Largest event I would count is 570; Baltimore SCG Open about a year ago, I went 7-3 on the day. (I enrolled in the more recent ~650 player open in Baltimore, but went 0-3 drop so I don't really count that.)
I think tactically, so my play decisions are all weighed carefully and I'm constantly re-evaluating my actions (though not really to the point of analysis paralysis - I almost never go to time anymore), which makes Magic kind of a stressful game in general; I've also got the personality of "hate to lose" in the sense that losing feels like I've failed in some way and it's directly my fault, so if/as the game turns against me I tend to stress out even more about what I may be doing wrong.
There's also a little social anxiety involved, since it's a lot of faces I don't know in the event hall - I try to mitigate this by being sporting and friendly with my opponents, so when they get all "game face" with me while trying to do that it tends to throw me off balance.
I think it's largely a personality thing; competitive events by nature foster a play to win atmosphere and that's not really what I want out of the game.
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