In terms of performance? None, assuming a well-written compiler. Depending on dialect you run into other issues though, such as the array size being part of the type signature, which is definitely not nice. The lack of adequate tooling and portability is another issue. Not strictly a problem of the language itself, but a problem you end up facing if you write low-level code in Pascal.

Don't get me wrong, I wrote low-level code in Pascal, too. It's nice and I probably wouldn't grumble too much if I had to do it again, but there's a bunch of stuff that comes in the same package with using something other than a language widely considered adequate for systems programming.