[p_l]

And the core website that the developers actually see while working on it might be just as fast as you imagine!

The developers are not in charge, usually, of what rules will be loaded into a tag manager by the marketing team (which might involve different sets based on URL or various other tracking data). The tag managers themselves and base ad auction and spyware stuff might be pretty performant, too - it's when you hit all those third party ads etc. that you might also see some shitty code.

But when many of those "features" are written assuming all the extra budget for themselves, well, things go bad fast.