Steps to reproduce

1. Create (or use) a survey with a moderate number of questions (our reproduction case: 81
   questions) and several hundred responses (our case: 516).
2. Go to Responses → Browse responses for that survey.
3. Observe page load time for a single page of 10 responses.
4. Enable the MySQL general query log while loading one page and inspect the query counts.

Actual behavior

Loading a single page of 10 responses took ~3 seconds, and the general query log showed the query

SELECT * FROM `questions` WHERE sid = <sid>

executed 526 times for that one page load (once per non-empty answer cell), each time loading every question row for the survey, only to run:

$found = false;
foreach ($rawQuestions as $rawQuestion) {
    $found = $found || (strpos($sFieldCode, "Q{$rawQuestion->qid}") === 0);
}

i.e. a linear scan just to decide whether $sFieldCode might be a question field, before optionally building the field map. Survey::model()->findByPk($iSurveyID) is likewise re-fetched on every call.

Expected behavior

The responses grid should not re-run survey-wide queries once per cell. Rendering N rows × M columns should cost O(1) (or at most O(rows)) survey/question metadata queries, not O(rows × cols).

Root cause

application/helpers/common_helper.php, function getExtendedAnswer() (called from SurveyDynamic::getExtendedData(), which is used as the CGridView column value expression for the responses grid — i.e. once per cell):

function getExtendedAnswer($iSurveyID, $sFieldCode, $sValue, $sLanguage, $question = null)
{
    if ($sValue == null || $sValue == '') {
        return '';
    }
    $survey = Survey::model()->findByPk($iSurveyID);
    $rawQuestions = Question::model()->findAll("sid = :sid", [":sid" => $iSurveyID]);
    $found = false;
    foreach ($rawQuestions as $rawQuestion) {
        $found = $found || (strpos($sFieldCode, "Q{$rawQuestion->qid}") === 0);
    }
    if ($found) {
        $fieldmap = createFieldMap($survey, 'short', false, false, $sLanguage);
        if (isset($fieldmap[$sFieldCode])) {
            $fields = $fieldmap[$sFieldCode];
        } else {
            return '';
        }
        ...

Patch example code provided, the performance regression is fixed on our local installation.