WAF with Form Based authentication slows down Website experience

In the past we used XG 17.5 with a WAF Rule to publish a Microsoft Product.
The users logged in on a Form Based Login with passthrough authentication.

This was fine.

A few weeks ago, we migrated the Microsoft Product to the latest release, these Product delivers the content in a different ways.
My developers says, it looks like a wafterfall in Browser debugging.

Now with this Form Based Login, the website slows down and is not usable for my customers.

Is there anything i can change or tweak in the form based passthrough?

Firewall is at 18.5.1 MR-1-Build326

Thanks

Jürgen