Apache delays serving an empty html file while processing a concurrent request

I have a website, which queries a Varnish server, which queries an Apache server, which queries a db server.

At 07:00:00, a request is send to the Apache server, which triggers a db request that takes over 30 seconds to process. While the db server is "locked", concurrent db requests are piling up, causing apache requests to pile up as well. So far, this is not my issue.

In the meantime, Varnish polls Apache every 5 second, with a 1 second timeout. The probe target is an empty html file.

Apache log tells me that every poll is answered with a 200 status code.

I get the following results from combined Varnish/Apache log :

Polled at   Served at   Delay (s)
07:00:26    07:00:26            0
07:00:31    07:00:34            3
07:00:37    07:01:01           24
07:00:43    07:01:01           18
07:00:49    07:01:01           12
07:00:55    07:01:01            6
07:01:01    07:01:01            0
07:01:06    07:01:06            0

What I don't understand is the following :

• Given that Apache serves every polling requests, it should means that the MaxClients has not been reached. Otherwise, I guess Apache would reject any new incoming polling requests. Am I right ?
• If Apache can accept connections for the polling requests, why is the response delayed ? Serving an empty html file should be as fast as usual, even if many concurrent requests are still waiting for the db to "unlock". The timing looks like Apache needs somehow the db to unlock, and other processes to be served, so it can process the polling request.

The delay causes Varnish to believe that my server is "unhealthy", thus causing automatic rejection of all following requests, while they could all be served within a 30 seconds delay.

Varnish config :

backend foo {
    .timeout = 60s;
    .probe = {
        .url = "/check.html";
        .interval = 5s;
        .timeout = 1s;
        .window = 10;
        .threshold = 8;
    }
}

Apache configuration :

Timeout 300
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 15
<IfModule mpm_prefork_module>
    StartServers          5
    MinSpareServers       5
    MaxSpareServers       20
    ServerLimit           200
    MaxClients            200
    MaxRequestsPerChild   0
</IfModule>

Don't hesitate to ask for more configuration informations or logs.