This is a topic that will be familiar to anyone who has experience in the world of Virtual Pipe Organs.
Latency on an organ is the time between you pressing the note and hearing it, usually measured in milliseconds (ms). Typically the faster the piece you are playing, the more latency matters - you need a responsive organ. It can be the difference between whether you can play that Toccata or not.
How does Box Of Stops cope with latency?
Box Of Stops has been designed with Latency in mind.
On a Raspberry Pi 2, with 8 organ stops pulled, typically you can expect a note to respond fully in about 10 milliseconds. Technically this means the Midi note_on message has been transmitted on all 8 Midi channels.
To relate this to a normal organ playing experience, something like a 50ms 'attack time' would normally be deemed acceptable with a tracker action, though clearly this will always be a bit subjective. (If interested The Physics Of Organ Actions talks about this topic some more).
If you would like to review the latency you are experiencing in Box Of Stops, please take a look at the Logging output.
What are the latency factors?
Everything that has to happen between you pressing the note and hearing it is a factor in how responsive the organ feels to play. These are the main areas that affect any virtual pipe organ:
Midi driver
Firstly some operating systems process Midi inputs quicker than others - how well does the USB/Midi driver perform?
Part of the reason Box Of Stops runs on a Raspberry Pi is because Raspbian is better than most here. With other virtual pipe organ software, it is often reckoned the Mac version will offer better latency than the same software on Windows.
Sound card
If a virtual pipe organ is using sampling, or works as a synthesizer, the performance of the sound card is crucial and a professional level sound card will make a big difference.
Box Of Stops is fortunate in this regard - it doesn't use a sound card.
When you press a note (i.e. send in a Midi message), it sends that same note back to the keyboard, or other output device, on several Midi channels. This does mean that strictly the performance of the sound production in your keyboard is a factor, but this is just the same as playing the keyboard normally.
Choose the right instruments
One other factor in the perceived latency of playing Box Of Stops is the choice of instruments on the Stops in your Registration.
Ideally you are looking for instruments with a short attack phase - how long the sound takes to go from zero to its peak level.
A classic example of an instrument with a slow attack phase is the Pan Pipes, with the lovely breathy start to each note. However mixed in with other instruments with a quicker attack phase, the overall effect will sound quite 'raggedy'.