It seems like the solution ratings are first sorted based on runtime, and then if a tie, on the time of submission which does not make too much sense. Maybe it should be sorted based on memory usage if a tie on runtime?

Rating based on time of submission gives incentive to be the first to solve new problems.  I agree that including memory usage in rating makes sense though.

There is a bug.

If you have multiple submissions of same execution time but different memory usage.

The final solution ratings will be still showing the first submission even your second submission uses less memory at the same execution time.