Could this be related to the “Disable REST API” plugin?
Yep, that was it. See WordPress Popular Posts is not tracking visits, what’s going on?, fourth item from the list.
Why does the admin-ajax.php query for the plugin take so long? The initiator is wpp.min.js with a duration of 2000 to 3000ms
admin-ajax.php? The plugin doesn’t use that anymore, unless you’re using a very old version of WordPress Popular Posts? Or maybe you meant requests made from wpp.min.js to /wp-json/wordpress-popular-posts/v1/popular-posts?
In any case, couldn’t really say why these requests are taking so long to complete without more information. Things like site traffic levels, database size, active plugins, etc. can all have an impact on the response times of admin-ajax.php and/or the REST API. You’ll want to review your current site configuration and see what’s causing the bottleneck.
On WPP’s end, see WordPress Popular Posts is slowing down my website, what can I do? for more details and recommendations.