For better and for worse, consumable XP boosts return in Battlefield 6. Activate one in the top right of the multiplayer menu, and you can double your account XP for a set amount of time—either 15, 30, or 60 minutes a pop. The timer on XP boosters stacks, meaning you can activate multiple boosters at a time if you're in it for the long haul.
There's also 'hardware' XP boosters, which are slightly less obvious, but these will double your progression with weapons, vehicles, and the like. With how long it can take to level up gear, hardware boosters are just as important as your regular career XP boosts.
While you're warned that XP boosters count down in real time, it's not obvious that this means literally right away. As soon as you activate an XP boost, it'll start the ticking clock, whether you're actually in a match or not.
I was pootling around in the main menus, checking out all the guns, so I had to immediately queue up for a match like a firefighter called to a blaze. By then, it was too late—I'd already wasted precious minutes from a 15-minute booster just loading in. It hurts more knowing it was given as a pre-order incentive, too.
My advice to you is to only activate XP boosts when you're in a match, loadout sorted with boots on the ground. Any time spent fiddling around in menus is time wasted once you've activated one.
Is it stressful to feel like you have to perform your best to make use of such a short duration? Yes. In my opinion, these boosters should only begin to count down when you're blasting other players in a match. It's cruel, honestly, but I don't make the rules.
Alongside these consumable XP boosters, there's also XP 'modifiers'. You'll get bonus points from playing with others in a party (you don't even have to be friends) and +5% for owning Battlefield 6—which I assume would only ever be disabled if you were playing the suspected free-to-play [[link]] battle royale mode.