Your first few levels in Splatoon 2 will fly by, but soon enough you'll be slowly crawling toward your next rank and weapon unlock. Fortunately, there's an easy way to speed things up.
If you visit Crusty Sean near the back of Inkopolis Square, you can buy special food that increases the EXP or money you earn from competing online. To get food, you'll have to spend tickets, which you earn for a variety of in-game actions. Tickets don't seem easy to come by at first, but there's actually a pretty simple way (via Reddit user Jaicera) to farm a bunch of 1.5x EXP tickets.
You'll have to have cleared the first sector of singleplayer missions over in Octo Canyon. Leave Inkopolis Square and head there, and make your way to the #6 hole in the second sector. It's located here, if you still have to find it:
Immediately after landing, knock out the boxes blocking your way and jump down to the lower platform. Then jump across to the even lower platform, so that you're looking down on a box that's off to the right:
Shoot the box and there'll be a 1.5x EXP ticket inside:
Collect the ticket and give it to Crusty Sean for a 1.5x EXP boost on your next 20 online matches. If you return to get a second ticket, you'll find it greyed out. You might think that means you can't collect it, but you actually can. That means you can quickly warp to and from the ticket's location, saving up dozens of tickets for later use.
There's also apparently a gold boost ticket you can farm near level 27 in sector 5 (source), but I haven't unlocked that sector and thus haven't checked for myself.
It's unclear if these tickets are working as intended or if Nintendo just failed to notice that players can still collect greyed-out tickets. It's also unclear whether Nintendo might patch the game in the future, or whether players who collect a ton of tickets might lose them or otherwise be punished.
Given that this "exploit" can be accomplished by straightforwardly playing the game and without resorting to any trickery (glitching through walls, disconnecting, that sort of thing), it would surprise me if there were any negative repercussions for grabbing a few extra tickets, but you never know. Regardless, I hope this is helpful for those of you who want to level up a bit faster.