It was significantly worse than playing on a console w/ a 4K TV (I tried it on my 4K TV). I'd guess 100ms+ added on.
No one playing competitively is going to bother, which will undermine their strategy to tie-in with streamers.
That's a shame, but it still was a beta. The guy playing on stage seemed to have significant lag and some eagle eye users have posted footage on twitter, very similar to what was shown of xCloud at Inside Xbox with that woman playing Forza. I think it's really telling that we are finding evidence (not proof) of less than stellar performance at this stage. I guess we'll just have to see when it releases this year.