Steps to reproduce:

1. Whenever there are quite a number of active events, widget Problems By Severity significantly degrades. Turning debugging in my case show that out of 180-200 seconds only 1.5 seconds is spent in SQL. System profiling shows that php-fpm utilizes the CPU a lot. Invistigation in the code showed that the most time is spent in blocks.inc.php

2.  
   $actions = getEventsActionsIconsData($problems_data, $data['triggers']);
    (blocks.inc.php line ~263)

3. which in turn led to
   getEventsActionsin actions.inc.php, line ~1487

pproblematic code is 
foreach ($events as $event) {
....
$event_actions = getSingleEventActions($event, $r_events....
....

at lines ~1530

So, the problem: this code loops through all the events and calls getSingleEventActions function for each event.

Inside getSingleEventActions there is a loop on all alerts.

So in my case, there were about 1k of events and about 200k of alerts, which led to total loop count almost of 200M.

So I suggest a simple fix by making alerts array indexed by eventd which speeds up the widget at least two-fold.

In my case simple fix did this:
$alerts_by_event=[];
    foreach ($alerts as $alert) {
        if (!in_array( $alert['eventid'], $alerts_by_event, FALSE)) $alerts_by_event[$alert['eventid']]=[];
        array_push($alerts_by_event[$alert['eventid']],$alert);
    }
 

And inside the loop call
$event_actions = getSingleEventActions($event, $r_events, $alerts_by_event[$event['eventid']]);
 
Result:
100-200x performance increase, time spent in php drops from 180-200 seconds to 1-2 seconds

Let me put fixed code altogether
$alerts_by_event=[];
    foreach ($alerts as $alert) {
        if (!in_array( $alert['eventid'], $alerts_by_event, FALSE)) $alerts_by_event[$alert['eventid']]=[];
        array_push($alerts_by_event[$alert['eventid']],$alert);
    }

    foreach ($events as $event) {
        //also adding empty alert arrays if there is no alerts for them
        if (!in_array( $event['eventid'], $alerts_by_event, FALSE)) $alerts_by_event[$event['eventid']]=[];

        $event_actions = getSingleEventActions($event, $r_events, $alerts_by_event[$event['eventid']]);
        $actions[$event['eventid']] = [
            'actions' => $event_actions['actions'],
            'count' => $event_actions['count'],
            'has_uncomplete_action' => $event_actions['has_uncomplete_action'],
            'has_failed_action' => $event_actions['has_failed_action']
        ];
        $mediatypeids += $event_actions['mediatypeids'];
        $userids += $event_actions['userids'];
    }