Be tactful and kind, but straightforward about what you can't/don't want to spend time reviewing.
"Thanks for the effort, but my time and energy is limited and I can't practically review this much code, so I'm closing this PR. We are interested in performance improvements, so you are welcome to pick out your #1 best idea for performance improvement, discuss it with the maintainers via ..., and then (possibly) open a focused PR which implements that improvement only."
"Thanks for the effort, but my time and energy is limited and I can't practically review this much code, so I'm closing this PR. We are interested in performance improvements, so you are welcome to pick out your #1 best idea for performance improvement, discuss it with the maintainers via ..., and then (possibly) open a focused PR which implements that improvement only."