I've never really had a tournament where I've lost more than 25 points. I think my worst performance was 0.5/3 against roughly equal opponents.

Best tournament performance I had was in a my club championship. Started playing when I was 21. Played until I was 25 and then plateaued around 1800. Everyone in my club was either expert strength or >1650 and I was stuck in a class by myself. I stopped playing for 2.5 years, except for 3m games online.

After the break I came back to the club and won my first 7 games. Everyone was under 1700 so I didn't think much of it. Entered the club championship rated 1780 and finished 9.5/12 (+8 -1 =3) for shared first, where my only loss was to a 14 yr old 1500 which I had a winning position and got very careless in the end game.

I was +1 -0 =2 vs the 3 experts in the field and +1 vs the lone master. Also was +5 -0 +1 with black and increased my rating by 120 points. That was about 3 years ago, and I only get about 4 or 5 rated events per year in now, but I've steadily increased my rating to 2096, never having more than 1 consecutive tournament where I lost points since I took that break.

One of my best was surely the 1983 Greater Boston Open.

In the first round, I won as Black in 25 moves in a Caro-Kann Two Knights from the top-seeded player, ~2500. Round two: lost to a 2360 in a 2.d3 CK. Nice position in the middlegame, but let slip and eventually went down to defeat. In the third round, beat a 2100 player, then beat a player ~2350 as White in a Nimzo.

Worst performance? Had some turkeys, none of which stand out.