In summary, I’ve explained my suggestion here:
https://forums.505go.com/discussion/77132/cards-requiring-different-amounts-of-experience-to-master-depending-on-event-investigating#latest
Let cards gain 20 XP in TG for the first four wins. After that (after getting all the rewards) subsequent wins can be awarded with 6 XP per win.
With the old system, I was able to master a new legacy mythic in 40 matches. We the current 6 XP system, it takes forever, i.e. 134 (!!!) matches. Needless to say, I stopped playing TG, until this rebalance is implemented.