3 widgets make very slow page

unfortunately no , our plugin does not cache database query , best thing you can try is the object cache , but you may need to modify a bit the widget source code to utilize the object cache if it doesn’t use it by default.

basically , you just need to use wp_cache_set() or wp_cache_add() , and wp_cache_get() functions

in place where your widget does the query , just imagine here

$result = query blah blah blah; 
wp_cache_set('widget_object1', $result, 'widget_cache', 3600); // this create an object cache with name widget_object1 , with TTL to 3600 seconds

and then from where your plugin uses this query result

$result = wp_cache_get('widget_object1');

to retrieve it from object cache directly , instead of run database query

ref: https://developer.wordpress.org/reference/functions/wp_cache_set/