LÖVE

In most languages/engines/frameworks, I could just write the movement code like this:

x = x + (speed * dt)

And it would work fine. But in Love2D, the dt changes nothing. If it's at 60FPS, it will move at a certain speed. If it's 20FPS, it will be much slower. What can I do?

You could show us some actual code, or even better upload a working .love file that showcases your issue; the snippet itself looks fine.

if you put dt as a parameter into the love.update callback (or whatever function you put that code in, while passing love.update's dt into it) it should work. As Nixola said, post a bit more code.

In my experience, dt has always worked as it should. As was said, provide an example that gives us the result you're seeing and we'll tell you if anything is wrong.

Also maybe your system specs.

My guess would be you're using the dt of the update function inside the draw.

As has been mentioned, as long as you pass dt into every function you place in update that has anything to do with movement or speed or whatever, it should work fine. And don't try to use it in love.draw as it won't work, but you can make it work if you absolutely have to by either calling love.timer.getDelta() manually or rewriting love.run to pass dt into love.draw as well and treating it the same. If you have to.