PSA: Don't ignore fast upgrades now, they're extremely good to split the cards you want for no extra cost!
Don't see too many people talking about this: Fast upgrades used to cost extra credits in order to buy boosters. THIS IS NO LONGER THE CASE. It now does not cost extra credits, it just costs the normal amount to upgrade the card and you get the boosters for free.
So if you see the card you like in Quick Boosters section, there's no reason not to get it.
Here's the kicker: if you don't want a card to show up in this section, accumulate enough boosters for the card and DON'T upgrade it. Only leave the cards you want to focus on low on boosters. Those cards will keep showing up in the Quick Boosters section and you can very quickly upgrade them by getting 40/50 boosters for free.
tl;dr: upgrade cards you like, don't upgrade cards you don't care about. Don't ignore this section of the shop, it went from trash to amazing.