Kontakt is my go to sound source for almost everything!

To answer your C1 to E1 question you will get a tail off using the Kontakt KB as it take a little time to move your mouse from one key to another. But with a MIDI controller you can change the notes at precisely the right time and not get that tail off. Or in a piano roll mode you can write in a C1 and a E1 at the right places.

A little more information about your setup would help tremendously. For instance what DAW are you using? Do you have any kind of MIDI controller such as a guitar or keyboard MIDI controller? What is your audio interface (sound card) and are your drivers for it ASIO or MME? Most of the time you will need ASIO drivers to eliminate latency. Are you using a 64 bit or a 32 bit OS? The more information you can provide the easier it will be to give the correct advise.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware