Thanks for the message.
This could be caused by a few things that QM can’t (or currently doesn’t) measure:
- A lot of slow PHP processing.
- Server-side HTTP requests that don’t use the WordPress HTTP API (for example, a direct cURL call).
- Reading a lot of data from the filesystem on a slow server.
Unfortunately the only real way to debug issues like this is to start using some lower-level profilers such as Tideways. I’ve listed some options here: https://github.com/johnbillion/query-monitor#related-tools
If you’re willing to get your hands dirty, QM includes some helper functionality for adding your own profiling to parts of your code: https://querymonitor.com/blog/2018/07/profiling-and-logging/. If you’ve got a vague idea where the problem might lie, you can start putting qm/start and qm/stop actions in the code of the form plugin to try to narrow down the problem.
Otherwise, your best bet is just to get in touch with the author of the form plugin and see what they say.
Bear in mind that this slow loading might be network-level or client-side too. Check the numbers in the Overview panel in QM to confirm whether those 15 seconds are taken up generating the page server-side.
If not, then your problem might be at the network level, or it might be a slow-down client side in the browser.