Does listening to slowed-down Spanish help or hurt?

It helps, as long as you treat it as a temporary tool. Slowing Spanish audio down makes it easier to separate words and catch sounds, which is useful at the beginning. It hurts only if you stay at slow speed forever, because real conversations won't wait for you.

Why slow audio helps

Real Spanish sounds fast to a beginner because words run together and unstressed sounds shrink. Your brain hasn't learned where one word ends and the next begins. Slowing the audio stretches those boundaries so you can hear them.

It also gives you a little extra time to process meaning. A sentence you couldn't follow at full speed may suddenly be clear at 0.75x. That success feels good and builds confidence.

Where it can hurt

There are two real risks:

Distorted sound. Very slow playback can make audio sound mushy or robotic, especially below about half speed. You may be training your ear on something people never actually say.
Dependence. If every audio is slowed, normal speed will feel like a shock. Your listening speed needs to catch up with reality, not just stay comfortable.

For why the real thing feels so fast, see why Spanish speakers talk so fast.

A simple way to use it

My approach is to use speed as a ladder:

First listen: slowed down (say 0.75x) to catch the words and understand the content.
Second listen: normal speed. Now you know what's coming, so your ear can match the sounds.
Later: try a bit faster than normal. Listening at 1.1x or 1.25x makes regular speed feel relaxed.

A moderate slowdown works better than an extreme one. Slow enough to help, not so slow it sounds weird.

Slowed-down speech versus slow speakers

There's a difference between a recording slowed by software and a person who speaks slowly and clearly on purpose. A teacher or learner podcast host who slows down keeps the natural rhythm and intonation. Software slowdown keeps the same rhythm but stretches it. Both are useful. Clear slow speakers are slightly better as a base, and speed control is a nice extra when you need it.

I built Langtok, so I'm biased, but it has a playback speed control from 0.25x to 1.75x for exactly this ladder. Any player with speed controls works too.

When to stop slowing things down

If you can follow most of what you hear at normal speed, you don't need it anymore. Keep it for harder material, like a fast speaker or a new accent, and use it as a rescue when you get lost.

The goal is understanding Spanish at the speed people really speak. Slow audio is a bridge, so use it, then cross it.