My idea is a "show more" button or page system so you only see 100-200 adopts at once.
Same thing could be implemented in the GTS when offering on trades.
I will make some example images because I am bad at explaining xD
Example 1: Only 100-200 adopts are shown at a time when opening any users box. You can click the "show more" button to see more (or show all?).
Spoiler:
Example 2: Similar to Example 1 but as a page system. 100 adopts per page and you can click through them.
Spoiler:
Example 3: Same as Example 1 (or 2, if this works better) but when offering on trades.
I tried to offer on a trade with Bulbasaurs yesterday but my phone didn't like the massive trade box.
Spoiler:
There is also already a suggestion for this kind of "show more" thing for trades in general.
Check that out too!