Latency is the time interval between the performer triggering a note on an instrument and hearing the audio corresponding to that note. The expected latency of a typical ChordEase setup is between 5 and 8 milliseconds. By far the largest share of this latency is incurred by the MIDI hardware protocol itself, which takes between 2 and 3 ms to send a single message. Because each note has to take a roundtrip from the instrument to the computer and then back to the instrument, the latency resulting from MIDI hardware alone is in the range of 4 to 6 ms.
Most modern MIDI interfaces use USB, and this may add an additional delay in both directions, depending on the USB driver's implementation. The MIDI instrument may also introduce delay between pressing a key and sending the corresponding Note On command, and between receiving a Note On command and playing the corresponding audio. ChordEase also has computational overhead, but it's comparatively small and doesn't contribute significantly to latency, provided ChordEase is running on a modern multi-core CPU, with minimal competition from other applications. Here are some things you can do to reduce latency:
Note that using a software synthesizer as your output MIDI device may increase latency, sometimes drastically. The Microsoft GS Wavetable Synth that ships with Windows is particularly bad: it exhibits latency on the order of 250 milliseconds, which makes it unusable except for testing.