SE had very high standards for ads on Stack Overflow, ads were curated, generally static images and well-behaved. I found the stance SE took on acceptable ads admirable, and they certainly left some money on the table by keeping the standards this high.
There are now ads on other sites in the network now, and those are not manually curated. These new ads are really far from the kind of ad allowed on Stack Overflow.
- they often load incredibly slowly (~2-3 seconds after the page itself has loaded)
- they're not relevant to the content of the site
- they contain animations or even full videos, not subtle animations but often quick and distracting flashy animations
- most cause around 5-8% CPU usage on my notebook continuously after being loaded. Some of the more animated ones cause 50% CPU usage while on screen, and they animate indefinitely (screenshot of one example with Chrome task manager visible).
- they make lots of requests (>75) and are often larger in terms of transferred bytes (0.5 - 1 MB) than the actual page content
- they use up a lot of memory, especially since Browsers like Chrome put different sites into different processes. See this example on Arqade of a single ad that uses around 190 MB of RAM
- the topics of the advertisments are sometimes a bit dubious like online gambling
- some users encountered very scammy ads for pseudoscientific or otherwise questionable content (those were removed from circulation after complaints on meta)
A single ad in the sidebar is usually larger in terms of bytes transferred over the network and uses more memory than the entire rest of the page together. I don't even want to know how this looks once multiple ads on a site are enabled.
Which ads you get depends on a lot of factors, so the experience might vary a lot for different users. This is what I saw from Germany in an incognito Chrome session, mostly on Workplace and Arqade refreshing the page a few times. Animated ads were pretty frequent for me, the heavy resource-intensive ones were rather rare.
It't a bit sad to monitor the page load for an SE site in the dev tools network tab where the actual site is heavily optimized and loads in 15 requests, and then there's the waterfall of almost 100 requests for a single ad that take seconds to complete.
These ads certainly don't meet the standards SE set for themselves in the past, ads were supposed to be "excessively considerate, ludicrously on-topic". So either the standards have changed, or the ad providers don't meet the request standard, or both.