What sometimes happened to me was, that when loading a foreign template for Cubase, the sound driver was set to some generic type, which I didn´t notice at once..

When latency began to be a problem, you check the buffer sizes of your audio card and everything looks fine, although you can notice a big lag.

Check if you definately selected and activated the right ASIO driver.

Other things to check : Do you use an effect chain, perhaps convolution reverb or an external effect processor ( TC, UAD, SSL )  ?

What happens if you click on the virtual keyboard with the mouse instead of using a Midi keyboard ?

Can you go down to 128 samples without drop outs ?

256 should also be O.K., but maybe a little bit on the edge if you are critical.

Someone already tried UAD in VE RPO 5 inserts? I have latency.. :( Other plug-ins (waves etc) - not latency.

LASS and other complicated K4 libraries that use input scripting are latent-period, no way around it. If you think about it, what LASS is doing with its auto arranger is taking your input and THEN deciding which samples to send what notes to. 

K4 is a horribly latent platform. As someone who came from GIGA with its kernel mode drivers, I can barely use it for "live" playing. To play the Scarbee EPs, I have to set my system on 32 samples--still doesn't touch my Kronos, which you can say I bought out of utter frustration with everyone developing for Kontakt. 

If you want to test what I'm saying, load K4 stand alone. Make sure you have the same buffer settings as your project was, ..load the same patch and play.

popmann, and what are you think about latency uad plugins in VE PRO 5 inserts?[*-)]

I don't use either, but obviously, a pci accelerated plug in in the insert of anything will cause huge latency increases. 

But in Cubase 6 uad plugins work without any latency.

Actually, they don't. Cubase does what's call "plug in delay compensation" or pdc. So, when you insert it onto an audio track, it is calculating the latency and sending the signal to the UAD card that much earlier so that it returns at the correct time. If you insert it on a real time played instrument like VInstruments, you will get the same delay. 

Now, it could be that VEP does not do PDC, or doesn't do it as accurately. That, I don't know, not being a VEP user. But, what cubase is doing is compensating-you will get the latency of you apply it to a live input. 

VE Pro 4 doesn't have PDC inside instances. VE Pro 5 does however, and very reliably too.

Karel, again please! - VE PRO 5 have PDC (like PDC in C6)?

Yes, that's exactly what I'm saying :)

Thank you very much Karel! One last question - how I can turn PDC for UAD plug-ins, opened in VE PRO 5? :)

Try with Logic buffer set to 1024. 

If it is still not working try to narrow down the problem suppresing instruments one after one (Pay in first)

Did you look at the processor load ? ( You can install BigTop that is in the develloper directory on the Mac os DVD, it's a very good free software  that draw graphic of many event  of your computer)