If you add bounds checking, then what happens is that on every index into the array, a line of code gets called that says "is the index the user specified less than the length of the array". On it's own, that's not a problem, but when you're doing this repeatedly millions of times, it's a performance killer.

Still, I would expect the overhead to be moderate, not a killer.

Bounds checking can block the ability to auto-generate SIMD, which then has a larger effect than the operations of the bounds checking itself.

Check this out. https://docs.julialang.org/en/v1/devdocs/boundscheck/