Please help us help you and fill where relevant:
Your LimeSurvey version: 6.4.12
Own server or LimeSurvey hosting: hosted on university servers
Survey theme/template:
==================
After upgrading to the newest version I've encountered a peculiar issue. Once I add participants to the survey, the survey settings and overall the back end part slow down significantly. The scope of the issue depends on the number of participants, I've tested about 5k to 25k participants and the more participants the slower system response.
Here's everything I tried:
- problem seems to exist both on surveys created before and after the update and regardless of how long or complicated survey is
- I checked PHP version -it's 7.4, so it isn't the source of the problem
- waiting for the system to catch up and/or deleting cookies didn't fix the slowness, only removing participants table did
- integrity check didn't show any problems with participants

Also one weird thing - trying to display more participants on a page works fine for up to 2500 of them, but then at 5000 system throws error 500.

Any ideas on how I could fix it?

Really interesting situation !

Where happen this slowness . All admin page ? page related to participant ? Public survey part ?
If you can : can you activate debug + debugsql and check if we load ALL participant table ? www.limesurvey.org/manual/Debug_mode
Do you have any external plugin ?
Best seems to report the issue : community.limesurvey.org/bug-tracker/

Hi Denis,

I updated to the 6.4.12 and also experiencing the slow loading not only on loading survey participants but also the survey itself. When I try to turn on debug and debugsql. I'm gettting the error belowAttempt to read property "folder" on nullC:\inetpub\wwwroot\limesurvey\application\models\Template.php(490)478 return $aTemplateList;
479 }
480 $aTemplateList = [];
481 /* Get the template name by TemplateConfiguration and fiolder by template , no need other data */
482 $criteria = new CDBCriteria();
483 $criteria->select = 'template_name';
484 $criteria->with = ];
485 $criteria->order = 'template_name';
486 $oTemplateList = TemplateConfiguration::model()->with('template')->findAll($criteria);
487 $aTemplateInStandard = SurveyThemeHelper::getTemplateInStandard();
488 $aTemplateInUpload = SurveyThemeHelper::getTemplateInUpload();
489 foreach ($oTemplateList as $oTemplate) {
490 if (isset($aTemplateInStandard[$oTemplate->template->folder])) {
491 $aTemplateList[$oTemplate->template_name] = $aTemplateInStandard[$oTemplate->template->folder];
492 } elseif (isset($aTemplateInUpload[$oTemplate->template->folder])) {
493 $aTemplateList[$oTemplate->template_name] = $aTemplateInUpload[$oTemplate->template->folder];
494 }
495 }
496 return $aTemplateList;
497 }
498
499 /**
500 * @return array
501 * TODO: replace the calls to that function by a data provider based on search
502 */

MS Server 2019 (IIS)
MS SQL 2019

Arg ! Another issue block the SQL part.

1. You can report this issue (you have a broken template ?)
2. Can you test with debug=1 or 0 but leave debugsql ?

The slowness happens on all settings of the survey with the added participants but not on the list of surveys.

I don't have external plug ins.

I din't run debug, but I noticed system was throwing some weird 500 errors when trying to show more participants or send invites once I loaded the participants, so by per your recommendation I reported a bug.

Thank you for the advice.

Thank you for your report : bugs.limesurvey.org/view.php?id=19486

My server speed are more quick then seems less clear, but i can confirm the 2A issue
About 2B : server issue in my opinion.