I've tried using the current version of Sheets App Script for some advanced business functionality, but it's horrendously slow - taking over 10 seconds to loop

I've tried using the current version of Sheets App Script for some advanced business functionality, but it's horrendously slow - taking over 10 seconds to loop over just 5000 rows. And they're using an ancient version of js, making development annoying. I really hope that this announcement would herald a renewed interest on their side in improving Sheets as a whole.