I'm still using the same PC I built in 2011 as my main development machine. It runs great. I never thought it would last this long but I'm still happy with the way it performs in nearly every way. Unity has gotten noticeably and significantly slower over the years. I can't say that I've noticed my PC performance getting significantly worse over the years with any other application I use. A couple years ago I started upgrading old projects to newer Unity versions when I realized the WebPlayer builds were no longer supported by any browser and I'd have to upgrade them to make WebGL builds. After I did that, Unity announced the LTS releases and I figured I might as well try to update them once per year to the latest LTS so they could stay current and playable as technology changed over the years. So for the past 2-3 years I've been taking projects that used to compile and build quickly when I originally worked on them in Unity 3.x, 4.x, or 5.x, and upgrading them to newer versions of Unity. Compile and build times have definitely suffered with each upgrade. The editor experience as a whole has suffered with each upgrade. All on the same PC I've had for nearly ten years, on which I haven't noticed significant slowdown otherwise.