Having used it in a major project its a shame that its so inefficient because the user experience really is much much better. It feels like a successor language to CSS because it fixed lots of unobvious and badly named attributes and makes lots of things just easier.
The code is more verbose but also you can completely understand how the page will look just by reading the html.
That said it makes sense the performance is so much worse, where you would have matched on one class for N styles you now match on N classes for N styles.
Theoretically its totally possible to do that matching at compile time and 'compile' the string of classes you wrote into individual ones per element for each combination used in the html though.