Some moments deserve more screen time, and some need to be over before the audience loses interest. A clip shot for one purpose often serves the other perfectly well once its speed changes. That is a two-minute adjustment here, with no re-shoot.
What it actually does
You give it a speed factor and it rebuilds the clip at that rate. The maths is simple: half speed means the finished video runs for twice as long, double speed means it runs for half the time. The tool is explicit about one consequence of this, and it is the one people notice first. Audio is preserved, and it follows the video, so slowing a clip down also drags the pitch down with it.
That is not a flaw in the tool. It is what a genuine speed change sounds like. If you have ever slowed a tape or a record and heard it go hollow, this is the digital equivalent. Correcting pitch is possible, but it is a different feature with different trade-offs.
Reading the result
The new duration is shown once the export finishes, and it is worth sanity-checking against what you expected. A 40-second clip at 0.5 becomes 80 seconds; at 2 it becomes 20. If the numbers look wrong, the setting probably is not what you think it is, which is much easier to catch here than three exports later.
Where the settings bite
Slowing down spreads the original frames over a longer timeline, which makes the motion smoother but also makes every frame more visible. Very slow footage, especially below 0.5, starts to look soft and slightly stuttery because you are asking a limited number of frames to carry a long duration.
Speeding up has the opposite trade. You lose frames rather than gaining them, so fast action can look jumpy, and very short results are hard to use. Between roughly 0.5 and 2 almost anything looks natural.
Good next steps
Speed is often one edit among several. Once the pacing feels right, trimming the ends tightens the result further, and pulling the sound out separately gives you an audio track you can use on its own. Both follow naturally in the same browser workflow.
When the audio stops working for you
The pitch effect is the part most people would rather avoid, and there are two honest ways around it. One is to accept it, which is perfectly reasonable for anything comedic, deliberate or very short. The other is to separate the two jobs: slow the picture down here, and deal with the sound separately. Some editors combine a slowed video with an audio track that has been re-timed and pitch-corrected, which sounds completely natural but needs more than a browser tool can do. Where speech is not the point at all, the simplest fix is to leave the original sound and slow only the visuals. Whichever route you choose, decide before you export, because changing it afterwards means starting the export again.
A speed change is not free in storage terms either: a clip slowed to half speed runs for twice as long and is correspondingly heavier. If file size matters to you, do the compression first and the speed change afterwards.