Happens only when user is member of a group with 'Debug mode' enabled.
Possibly connected to ZBX-5735?
Anyhow, haven't taken a look at ZBX-5735 yet.

Now I'm pretty sure it's caused by over-stressing the browser.

Gotcha! I don't think it's intended that way:

--- SNIP Debug output ---

6. item->get [items.php:721]
Parameters:

Array
(
    [itemids] => 
    [output] => Array
        (
            [0] => itemid
            [1] => type
        )

)
--- SNAP Debug output ---

Now I've to find out if it's my fault...
All fits toghether: happens only when updating item(s) in template

Hmm... that's not good in case of a template item because $data['hosts'] will be NULL:

--- SNIP items.php ---

        // hosts
        $data['hosts'] = API::Host()->get(array(
                'itemids' => $data['itemids'],
                'selectInterfaces' => API_OUTPUT_EXTEND
        ));
        $data['is_multiple_hosts'] = count($data['hosts']) > 1;
        if (!$data['is_multiple_hosts']) {
                $data['hosts'] = reset($data['hosts']);

                // set the initial chosen interface to one of the interfaces the items use
                $items = API::Item()->get(array(
                        'itemids' => zbx_objectValues($data['hosts']['items'], 'itemid'),
                        'output' => array('itemid', 'type')
                ));

--- SNAP items.php ---

I confirm the behaviour for 2.0.8 and 2.2.1. It's only 3 seconds on my side though.

RESOLVED in svn://svn.zabbix.com/branches/dev/ZBX-7545.

The problem was that when trying to mass update items from a template, the frontend performed an API request on items.php:834 that returned all of the items. When debug mode was enabled and there were a lot of items, that caused a considerable lag in the browser and the form was rendered with a delay.

Marc, thanks for pointing out the problem, otherwise I would be having trouble figuring out what caused it.

When fixing the issue I've discovered a related bug - ZBX-7997, but it will be fixed in a separate issue in 2.3.0 only.

<richlv> if it's about application updating in templates, that seems to be a regression from this issue - worked just fine before r44119

jelisejev Hm, I was sure I was able reproduced the issue before. When, anyway, now it will be handled in ZBX-7997.

TESTED.

Fixed in 2.2.4rc1 r44119 and 2.3.0 r44120.

CLOSED.

(1) regression - can't add/replace applications (mass update lost those two checkboxes)

jelisejev This will be fixed in ZBX-7997. CLOSED.