I'm using Linux for my daily work for more than ten years now and develop for MacOS since around 2000, and I honestly cannot confirm this. If you have a fast and well-tuned machine, the sluggishness of modern applications might not be so notable, but it surely is there, and then there are also many usability issues of desktop software on Linux. Not to speak of browser-based applications, which mostly have unusable user interfaces anyway. For MacOS, usability is still high, but the multithreading and API layering on Cocoa is and always has felt sluggish to me. I no longer use Mail.app but remember it as a particularly bad example.
I agree with the OP, for actual use computers can do more powerful things than they used to be able to, no doubt about that, but programs and operating systems continue to feel slow and clumsy. Android in particular, but in the end all operating systems.
Bloated GUI frameworks, use of unoptimized images in GUIs, and non-optimal multicore programming are to blame, I guess.