SSE, what determines the return time, how serious is the load on the server?

Question

PHP

I started digging towards SSE to return game statistics.
I actually looked at this example:

public function actionIndex()
    {
        
        print('
            <div id="help"></div>
                
                <script type="text/javascript">
                var help = document.getElementById("help");
                
                var source = new EventSource(\'/site/sse/server/\');
                source.onmessage = function(event){
                help.innerHTML += event.data;}
                
            </script>
        ');
        
    }
     
        
        

    public function actionServer()
    {
        header("Content-Type: text/event-stream");
        $t = date("H:i:s") ."<br />";
        echo "data: $t\n";
        echo "\n";
    }

At the output I got:

04:38:32
04:38:39
04:38:45
04:38:51
04:38:57
04:39:03
04:39:10
04:39:16
04:39:23
04:39:29
04:39:35
04:39:42
04:39:48
04:39:54
04:40:00
04:40:06
04:40:13
04:40:19
04:40:25
04:40:31
04:40:37
04:40:43
04:40:50
04:40:56
04:41:02
04:41:08

Recoil 6 - 7 seconds. Actually questions:
1) On what time of the answer depends? Can it be edited?
2) if, for example, I need to count the number of users who are temporarily in the table:
select count(*) from table where status=1;
How much pressure will this case create? (well, there will be approximately up to 100,000 records, on average 10,000).