Really weird issue here. I occasionally use an Akai MPK mini with Sonar X1 on my laptop and I'm getting what sounds like latency when trying to use it with any soft synth. I use a Roland Quad Capture as my soundcard running with 5.8ms latency. However for me the problem only occurs when the transport is engaged, e.g. playback or recording - with the transport stopped I'm able to play soft synths with no problems. Has anyone else ever had this problem before and what can I do to fix it? The Akai is USB class compliant and other than the Quad Capture there are no other USB devices connected - the problem is even present using the internal soundcard with ASIO4ALL with 5.8ms latency too. MIDI Playback of softsynths from a project is not a problem - it's just from the connected keyboard....

Do you have any effects in the project?  Especially CPU hungry ones like perfect Space or "Look ahead" ones?  Switch these off when tracking or freeze them if you do.
 
You could also try the PlugIn Delay Compensation button, but this can make things play out of time.

Not sure what caused the problem. I loaded the project I had the problem with and removed a couple of plugins and added them back again, all sorted now.