It's not just the ads. Tag managers are the scourge of performance. Marketing folk just want to stuff them full with dozens of analytics services and third party integrations. And they want to do that without involving pesky engineers.
Some, like Google Optimize, tell you to to clobber performance by hiding all content on your page using CSS until their code has loaded and run.
Some, like Google Optimize, tell you to to clobber performance by hiding all content on your page using CSS until their code has loaded and run.