What matters more than anything is CHAMPION PLAYED AT SPECIFIC ROLE, with Creep score comin in close behind.
1: champion played, thier popularity, and their winrate
For example:
will always net an S+ at a lower performance rate than
.
2: Specific Role
Role Specific Champions going off meta (support more so than others) skew the scale a lil due to end game stats and mainly how poorly these champions usually do when taken out of their main lane.
For example: Take a non support champion
, into the support role, and you will receive high grades at far lower performance than say 
3: Cs numbers
Point blank if youre top or mid, youre gonna want close to 9 per minute, strive for 10(double what you stated in the post). If you play jg, less will suffice, and as for support, i talked about it above.