Create two page handlers one for outgoing and one for incoming survey request, so instead forwarding to the survey directly send them to a custom page, write some magic value to session, then forward them to the survey. When survey is complete send them to the incoming page, read the magic value (if possible, or somehow compare that survey is the one you sent them to), trigger an event and forward to landing

I did it! It was easy due to your tip. Thanks, Ismayil.

My site is hosted in a powerful server of my property, and no other host is located in it. So there is no external influence.

I have detected (observed) that some plugins produce a lot of CPU consumption, and that the number of queries to the database is very high for the number of users. So I want to measure the impact of the plugins individually.

Elgg 2.1 added a simple profiler that's only documented in settings.php, and reports data in the JS console. If you do this on an empty page (e.g. login) you can at least measure a baseline for core+plugin boot plus page generation. Ultimately Elgg is a vanilla PHP/MySQL app so the same testing/metrics/recommendations apply. Upgrade PHP first! You might look into Blackfire, but AFAIK no one on the core team has tried that yet.

Thank you very much for your help (@steve_clay).

I followed your advice. I tested the profiler and obtained a good experience. In the next time...I maybe will test with Blackfire. I will report about my advances.