Agile is not about being quick, it's about delivering what the customer actually wants. When you do Waterfall you gather all the requirements, then you sit down and code the thing. Only to find out months or years later that you delivered crap as the customer didn't even know themselves what they wanted.

With agile you take it one step at a time. What is important now? Get the requirements for this feature, deliver it in the next two weeks (or at least a part of it). Then the customer, which can be an actual customer, or your internal Product Owner, or a Product Manager looks it over. If the whole thing is perfect? Nice, carry on to the next thing.

Often you find out some detail was overlooked, or a new requirement came up, or the design didn't fully work out. So pack it into the next sprint and do it better. You'd never get this feedback if you gather "all" requirements first and then just try to go from start to finish.

Agile certainly has its upsides when done right, unfortunately there's not a lot of companies who manage to do so (like most I've been part of). Despite being messy at times, it's still better than Waterfall. There's too many meetings either way.