What is the fastest way to learn Spanish?

The fastest way to learn Spanish is to put in as many good hours as you can, every day, and make those hours count. That means focused study of the basics, a big volume of listening and reading at your level, speaking early, and spaced review so you don't lose what you learn. There's no trick that replaces the hours.

Speed is mostly hours

Every promise of "fluent in 30 days" is selling you something. Spanish is one of the easier languages for English speakers, but it still takes hundreds of hours to get comfortable. You can read about how long it takes for the numbers.

So the real question is how to get the most out of each hour, and how to get more hours into your week.

Get more hours without more effort

You'll find more time by attaching Spanish to things you already do:

Listen during commutes, walks, chores and workouts - see learning during your commute
Swap some of your phone time for stories, shows or podcasts in Spanish. Full disclosure, that's the idea behind Langtok, which I built: short Spanish stories you swipe through, with a streak to keep you coming back
Change your phone and apps to Spanish for small daily exposure

None of this feels like studying, and it adds up to a lot over a few months.

Make each hour count

Not all practice is equal. These tend to give the most for the time:

Listening to things you can mostly understand. If you understand almost nothing, it's noise. If you understand everything, you're not growing. Aim for something like 70-90%.
Speaking and writing, with feedback if possible. Producing language is the part reading and listening can't give you. A tutor or exchange partner a few times a week speeds things up a lot.
Spaced repetition for vocabulary. It lets you remember far more words in less review time. More on what spaced repetition is.
Learning the core words first. A small set of high-frequency words does most of the work early on.

What slows people down

Spending months on beginner apps without ever listening to real speakers
Waiting until you're "good enough" to speak
Studying hard for a week, then dropping it for a month
Chasing perfect grammar before building any fluency

I'd say the last two are the biggest. Consistency beats intensity, and a plan you can keep up for a year will get you further than a sprint you quit.

Is immersion faster?

Living in a Spanish-speaking place can be the fastest route, because you get hundreds of hours of input and forced speaking without trying. But it only works if you actually engage. Plenty of people live abroad for years and stay in an English bubble.

You can get much of the same effect at home by immersing yourself without moving abroad.

A realistic fast plan

If you want to move quickly, I'd do something like this:

1 hour or more a day, split into a couple of sessions
Half of it listening or reading at your level
A quarter on structured study and vocabulary review
A quarter on speaking or writing

Do that for six months and you'll be much further along than most people. Just remember that fast means more time on task, not a secret method.