Was on march 2020. Unless you change the mode you will not be eligible to backfill. Also when you backfill you get priority queue, which often creates a match more faster for you. So purposely trying to level up as backfilling in fact would make you ending in a game that you wouldn’t backfill.
Also from bnet:
" Quick Play
In Quick Play, you can leave during the Assemble your team screen without penalty. After play has started, leaving games before you see the Victory or Defeat screen will count towards leaver status, which results in a 75% penalty to all XP gains."
I found some mention on 3rd party site about explanation on older forums about it but not sure if it’s entirely true right now:
"
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If you leave too many games and your “games played” vs “games completed” percentage falls below our required threshold, you will receive a warning message (this percentage is calculated based on your 20 most recent games).
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If you continue to leave games after receiving a warning (which will display in the lower left-hand corner of your screen), you will incur a 75% EXP penalty on all future games.
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The penalty isn’t time-based, but instead will be removed once you complete enough games to raise your “games played” vs “games completed” percentage above our required threshold (again, this percentage is calculated based on your 20 most recent games).
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If you disconnect or are removed due to inactivity from a game, it will count the same as if you leave the game intentionally.
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If you leave a game before the “Assemble Your Team” phase is over (i.e. before you enter your starting spawn rooms):
It won’t count as a game played
You will not be considered a “leaver” in this scenario -
If you leave a game after the match ends (i.e. after you see the VICTORY/DEFEAT screen):
It will count as a game played—and if your team won, it will count as a win
You will not be considered a “leaver” in this scenario -
If you leave a game after the “Assemble Your Team” phase is over, but before the match ends:
It will count as a game played
You will be considered a “leaver” in this scenario
"
I hope it helps you to know the perks around it. Keep in mind that playing consecutive matches also gives you a xp boost, playing in a group increases also.
But by far the best way to farm credits and xp, that I found was flex queue matches on QP and playing in a group some sequential matches, if you don’t mind to be in any role or play any hero each new match. In roughly 3-7 matches you can level up and get 75-225 credits. That mostly would mean around 14-70 minutes