I believe it was with the Winter Update they actually buffed the XP gains in Meltdown by 50%.
Capture & Face-Off are also incredibly quick compared to most other game modes. I typically reach level 10 in 8-9 minutes in Face-Off for instance.
The main problem is that the games are simply too short for the most part. You become level 10 in Incursion more frequently than the other game modes purely because the games tend to last longer. Incursion arguably has the slowest levelling out of all game modes.
Another partial reason being how kill-reliant levelling becomes throughout the match. If you want to level up quickly, you need to kill the highest level opponents as the exp. you gain from kills are multiplied by their level.
I do wish though they would slightly increase XP gains in Incursion and reduce the kill-dependancy.
The other game modes just need to last longer, which could largely be fixed by thr matchmaking having a tighter ELO-range.