Concurrent PHP job issue on IBMi

Hello to everyone,
I’m experimenting some performance issues on an IBMi machine running PHP and I would like to ask if someone have some hints.

Some premises about the environment:
IBMi Power 8 with OS400 V7R2 on a IASP storage
Zend Server 8.5.1 with Z-Ray and many other services disabled, at the moment we just use the HTTP Apache service and the XMLTOOLKIT service.

We are running a PHP application which receives many concurrent ajax (asynchronous request) per user from the browser.
At this point we would expect that all the requests are handled in parallel processes.
But running the WRKACTJOB on the server produces just a few (one or sometimes two) actual working jobs as follows:

Subsystem User Type CPU Status
QSQSRVR ESCPHP PJ 2,1 RUN

We understood that the QSQSRVR Pre-start Job is the connection to the database and our application is structured to open one single new, non persistent, connection for each request.
It seems that our requests at this point are not handled concurrently, but re-using the same Pre-start Job even for different requests.
So the first Ajax requests give a fast and correct answer to the browser, but the last ones end up in a Timeout Request.

We already tried to do some tuning on the fastcgi.conf:
StartProcesses="1" => StartProcesses="2"
SetEnv="PHP_FCGI_CHILDREN=10" => SetEnv="PHP_FCGI_CHILDREN=40"

This operation opened more Apache/Zend Server sockets, but did not change the behavior of QSQSRVR
Subsystem User Type CPU Status
ZENDSVR6 QTMHHTTP BCI 0,0 PGM-php-cgi.bi TIMW

On a different server (a testing environment) we tried to enhance the number of Pre-start Job available for QSQSRVR using the following command and restarting the system, but we experienced the same issue:
ADDPJE SBSD(QSYSWRK/QSQSRVR) PGM(QSYS/QSQSRVR) STRJOBS(*YES) INLJOBS(50) THRESHOLD(40) ADLJOBS(20) MAXUSE(*NOMAX)

Is there some info that I’m missing or some other configuration that I could try?
Thank you in advance for your help
Have a nice evening

Cassiano Vailati - ESC