Since the MK Team hasn't been able to really able to answer online questions (mainly all the modes people are speculating, if there are spectators, if there is a tournament mode, etc.) as well as them giving the typical "there is no lag" sales-phrase, it's hard to really know.
However, Based on E3 reports from high-level players from Shoryuken.com see the online archetype for fighting game delay of inputs like this order:
- Dead or Alive Ultimate (supposedly 0 frames of input were lost, 60 input frames for 60 game frames, perfect. I highly doubt it and assume the game runs normally at 55/60 fps, but with Team Ninja delaying the game for over a year [was supposed to come out last october] to do all their online modes and code optimization, it might be the first true perfect online experience. Even if it's not perfect, it will truly be near perfect.)
- Mortal Kombat Deception (out of 60 fps, at least a few frames of input were lost. Therefore when you press something, there are about 45-50 input frames out of a game running at 60 frames. Expect a delay on average 3-5 frames for everything you press. It's very hard to tell if it's that good with MKD though since animation is very unfliud and also because 3d animation is a lot easier to think that there is no lag, although 45 input frames out of 60 game frames sounds about midway's level of skill and timeframe for development. Could be more slower based on unideal servers of course.)
- Capcom vs SNK 2 (Not even Kaillera quality. Kaillera players generally use 20 input frames out of 30 game frames. Therefore CvS2 has around 15 input frames for every 30 game frames.)
- Guilty Gear XX #Reload (Hella frame loss for inputs, so much that the actual GAME frames are constantly being skipped in order to retain game speed. Looks very choppy and its almost unplayable from the Japanese build since people were using standing dust like it was reliable for every character, even Baiken lol [slowest standing dust])

I read somewhere that the performance online will be excellent and that Midway would not have included the feature if they could not get it to play extremely smooth. I'm not too worried about it.

The funniest thing is every magazine and online source said Capcom vs SNK 2 virtually had no lag and every player who plays fighting games at all is like WTF? Same with Guilty Gear XX #Reload, man that shit is laggy (based on Japanese vids and early impressions of the US version). Its hilarious, don't take what the company says trying to sell their game, take what unbiased players say about how it works online.
Just cause it looks smooth (the game frames keeping a constant 60 fps) and responds okay, does not mean that you have as many input frames as displayed frames. If the game is running at 60 frames a second and you're missing 15-30 input frames every second, few people on this forum would be able to notice, even though everything they press is delayed by 3-10 frames.

HDtran
Thankyou thankyou thankyou.
That was exactly the kind of info I was looking for.
Hope to see some of you guys online!
-Vermi

You're very welcome.