When groups are enabled at the course level, the gradebook grader report page has the a group filter enabled, and this filter populates a dropdown menu with the name and icon of the group picture of each group in the course. Moodle added this version of the filter in version 4.3 https://tracker.moodle.org/browse/MDL-76139
Currently, the filter loads all of the course groups and group icons into a searchable dropdown list. This does not scale well in courses that may have hundreds or even thousands of groups.
For example, I have a user that runs a large course with small group tutors, so there are many small groups (~6800). When accessing the grader report, the browser commonly hangs and becomes unresponsive, and eventually the browser crashes with an out of memory error.
I have been able to replicate this on versions 4.3 to 4.5, with anywhere from 3,000 to 5,000 groups. I'm sure the exact number of groups that is a problem could vary with the end users browser and computer resources.
I have been able to work around the issue somewhat by adding custom CSS to either hide the groups filter altogether, or in less extreme cases not displaying the icons. But I think ideally this filter should be similar to the enrol users search on the participants page, where it will display up to some arbitrary number in a dropdown that match the search term.
/* don't display icon in the grader report groups filter */
body#page-grade-report-grader-index div.group-search img {
display:none;
}
/* or don't display filter at all */
body#page-grade-report-grader-index div.group-search {
{{ display:none; }}
}