Computerworld tests says: IE8 still in the last place about javascript performance, but its 70% faster than IE8 Beta1.

See the pontuation:
1 – Chrome 2.0.159.0 – 1275
2 – Firefox 3.0.5 – 3037
3 – Safari 3.2.1 – 3050
4 – Opera 9.63 – 4139
5 – IE8 RC1 – 5573

You can see more about the test here.