The "view all submissions" page in assignments loads very slowly when configured to use group submissions and when displaying a lot of users with no submissions. To reproduce:

1. Create a test course with 100 users in it.
   1. php admin/tool/generator/cli/maketestcourse.php --size=S --shortname=group-assignment-test-s
2. Choose an assignment in the course.
3. Edit settings
   1. Submission types -> File submissions -> check
   2. Group submission settings -> Students submit in groups -> yes
4. View all submissions
   1. Options -> Assignments per page -> all
   2. Options -> Filter -> No filter

=> The page takes a noticeable amount of time to load. For me it's about 8s (reported by browser's developer tools)

If you want a more pronounced effect do the same in a course with 1000 users (--size=M). In that case it takes around 8 to 10 mins for me.