Beyond the fact that mirrorless viewfinders lag a little, one sometimes sees the claim that shutters are slower to react on those cameras, too. I wanted to investigate this claim using data from imaging-resource. Here is the result (green is fast = good, red is slow = bad):
Many more comparisons would be possible using data about Sony’s SLTs, but I think the above is a good and reasonably comprehensive overview.
What can we glean from the data? If anything, there seems to be an advantage to using an SLR or SLT if shutter immediacy is what you’re after.
breakfastographer 
But mirrorless cameras can make use of prebuffering to offer what essentially amounts to negative shutter lag.
As far as I’m aware, that’s so far been implemented by only Panasonic and Olympus. I’ve also been told the battery drain is considerable. It’s one of the things I want to benchmark in more detail when I have the time and a suitable camera.