See Noam’s post in this discussion
http://www.soundgym.co/space/talk?id=NASZ2448MAR&t=V24HPBAOJCF

Noam Gingold
Aug 17, 2020
SPI is calculated by the 3 top scores in each game. The first score provides you x3 points, the second score x2, and the third x1. For each game, you will get higher game scores for higher levels, so when you pass levels you will get more points and your SPI will be higher.
When you reach a very high level in a certain game, you will still earn more points for the next level, but the multiplier will be a bit smaller than before. If you continue to pass levels in this game the multiplier will keep getting smaller. At some point, it is wise to focus on reaching high levels on other games to get more SPI.
The multiplier starts getting mildly smaller at level 21, and get a bit smaller with each level that you pass. This way we want to make it harder to get medals (Especially gold & diamond) based on only a few games that you are very good at.

Thank you @Romano G !