When I played things very fast I usualy ignored some slight details, play every thing faster than every body else & no body noticed.
I actualy learned this bad habbit from Slayer , and applied it to everything I played ... Metallica, Megadeth , Pantera & even Vai & Malmsteen.
I thought that this was the meaning of improvisation but I was damn wrong, cuz a realy good player may notice I'm not playing correctly, also I get sloppy when I try to play slower & anybody may notice that easily (I never played fast stuff slower then).
At some point I decided to play things the the real way they should be played, very precise & with max accuracy. So I had to go slower.
It was very hard & painfull, and took me a lot of time to correct everything that I played wrong, but now I feel happy that I play every thing very accuratly & faster than ever.
It realy worth the time & the pain.