I don't think that web is slow, unless you're running seriously underpowered computer. That's not the case for me. Web is not as fast as it could be. And that's because customer does not want to pay for fastest website in the world. It should be fast enough and then features matter. You can spend year making your website load in 0.1 second while your competitor will implement important features and his 2-second loading website will win.

And that's not really about web. It's about software in general. Discord launches in 8 seconds. Ventrillo launches in 1 second. But people will use Discord, because 8 seconds is not that bad and then features matter.

And that's not really about modern software, as old software was slow as well. Also it was very buggy. Windows 98 used to BSOD here and there, so you had a routine to press "Save" button every few minutes and keep few copies just in case. Emacs was expanded to "Eight Megabytes And Constantly Swapping".

There's some old software which survived till today and it's now wicked fast, because it was made for old hardware. But people still prefer to move on to new software, because of features or just shiny new design.