How do you measure the productivity of someone that can wade into a horribly buggy 10 year old piece of code that has terrible edge conditions and is integrated

How do you measure the productivity of someone that can wade into a horribly buggy 10 year old piece of code that has terrible edge conditions and is integrated with other 10 year old pieces of code that you can't break, and you need to work out how to fix it and test it? I often feel like I'm pretty slow in terms of my output, but I generally wade into really tough problems after other devs/teams have flailed at them for weeks and I manage to solve the problem. Does that make me a "10x programmer"? I don't tend to greenfield anywhere near as fast as that so I can't claim to have written some 40,000 line program from scratch in a month. I can write some 100 line patch in 2 weeks that nobody else could come up with though and keep some 10 year old 40,000 line production critical codebase still running.