I have for the projects where creatively it makes sense. It's a nice visual effect to add to the feel of the project.

Absolutely not!!!

First off they are slow loading, slow rendering, bandwidth wasting, battery wasting, and often involving images that to be frank are too damned big to have ANY business on a website in the first damned place!

Second they are slow loading, slow rendering, bandwidth wasting, battery wasting, and often involving images that to be frank are too damned big to have ANY business on a website in the first damned place!

Now I realize that is in fact just one thing, but it's such a colossal sticking point I felt it necessary to repeat it!

There's a lot of artsy-fartsy useless garbage being sleazed into layouts by people under the DELUSION that they are designers -- when they don't know enough about HTML, CSS, Content promotion, emissive colourspace, or much of anything else to be designing a blasted thing. Parallax is so often used to cover up a complete lack of content of value and to make sites more annoying and less useful, it falls into most of the other goofy scripttardery and CSS tricks that add nothing of value to sites whilst pissing on usability from orbit!

But again, I view design as being more about engineering than art... just as with electrical design, mechanical design and architectural design. You hire an "artist" to do that stuff, you end up with idiotic halfwit bull like the various "death ray" buildings worldwide.

NOT that said attitude is well understood by the mouth-breathers who usually go "I want a website" without actually understanding what one is or is for first -- all you need to do with them is make them go "ooh, shiny" to fork over a check whilst pumping them full of snake oil.

There's a reason the only place you see that crap is on sites for small businesses who don't know any better, brick and mortars for whom a web presence is a afterthought, and sites for the sleazy hoodoo voodoo peddlers to hock their wares.

There's a reason even deviantArt, a website for the art faygelahs doesn't pull any artsy fartsy shit with their design.

I'm actually doing one site that the client asked me to put parallax backgrounds on some sections.

What I've discover is that having a section with background-attachment: fixed; and background-size: cover; takes a lot amount of scroll speed and performance of the page. When I remove the background-size and attachment everything went again super smooth and fast and I haven't add a single line of Javascript.

Doing a little search and testing on the topic I read that removing either one (attachment or size) improves the performance, also it has to do with the image size. So what I'm about to test is if I resize the images and make it so that with responsive I change the bg image size if it removes the nerf of performance or will use parallax js to make the exact effect.

So, as a quick summary, don't use more than one section with pure CSS as effect of parallax background, and not even once if the image is bigger than 1MB.