I think the performance part of decoding is really about... interfering with other applications as little as possible.
My last system was a 4400+ and was CAPABLE of playing hd... but not very good at it. Even at 720p if you opened a new tab, or refreshed a page with a lot of pictures, or decoded a file...you get massive frame dropping. Every dropped frame feels like your stabbing yourself in the eye...ok, not that bad, but really freaking annoying.
Think about it this way. You say that your "high end" pc it takes "a couple of seconds" to decode, perfect... for the sake of argument I'll call that 3seconds while it decodes and 2 seconds for the player to catch up if your processor is really bad at hd. That is 5 seconds of the video dragging... thats pretty damn unbearable... now imagine that it happened every 30 seconds. You would probably get to the point you would leave the pc while it downloads. Now we're bringing back memories of dialup and slow pcs. Now even if you say that the people with slow pcs, probably have slow internet... what if it happened every minute... 5 seconds of torture every minute is still torture.
Also as I linked, there is a 64bit unrar.dll available now... and you cant just switch them out; a 32bit exe can not call a 64bit dll... they have to match. While unrarring on a fast pc, the limitation is really in your hard drive. However... it could mean the difference between extracting x.rar at 40-60MB/s while using 30% cpu and still doing the same speed but only using 10%... thats 20% more that can be used for video decoding! Granted, I'm making up numbers at this point, but in some cases (especially hd...even on fairly new pcs) every inch counts... even when you have more than enough.
PS. I have no performance complaints on my new 6400+ yet it STILL drags on 1080p during some scenes if the priorities aren't tweaked right... x264 is a freaking beast!
