When IE6 came out it was kind of a breath of fresh air... it allowed for a lot of things, and the v4 browsers finally fell off the map. IE 5.0.0 had some hideous bugs on stamped CDs (Office 2000, Windows 2000) that I had to work around for a couple years. IE6 corrected many of them.
Now as things progressed, IE6 became a boat anchor for a long time.. and IE7/8/9 though relatively current at release fell behind very quickly. IE10-11 were also rans in my opinion, and I'm glad most people get to ignore them now.
Nothing compares to dealing with IE4 + NN4 issues... it was truly painful and I'll take what we have today over either. Most people aren't dealing with most of the newer browser features... but for those that are, it can be bad. Safari is the worst actor in the bunch, and they are emphatically not rock solid on that front.
At least the early IE vs NN stuff was fast to change and interesting. IE6 issues were the exact opposite :(
It was entirely due to MS’s negligence, not just XP.
Not least most of what you mention (other than the 300ms delay) is OS level anyhow.