For large instances, it’s not “just a single query”. You also can’t miss anything, so photos and similar - if they have uploaded something.

So, you went from "all instances are liable" to "big instances won't be able to handle it". Not only you just moved the goalposts, you are also missing the point of the Lemmy devs: if compliance with GDPR is problematic only for instances that are so big to the point that the volume of requests can not be manually processed, then it's not something that should be a concern for the developers of the main software and the cost to implement such a thing should be born by the admins themselves!

Also, does your instance have a cookie prompt?

Cookie prompts are only required if you have tracking cookies, which I don't have on my website or any of the instances I run. Cookies used for authentication or basic functionality (let's say to store the user preference for dark mode) are not tracking the user across multiple sites and therefore do not fall into the requirements for disclosure.

Edit: downvoting without a response serves only to show how lost you are in your argument. You spent the best part of the last two days fueling the mob and throwing accusations at the devs and basically making them criminally irresponsible and now you can't even support the premise that EU instances are somehow not able to comply with the law.