Lag, in its simplest terms, is impaired functionality, due to many different factors that cause strain on a computer's processor (in this case). Lag can be reduced in many different ways, often by fixing what causes the lag in the first place. But keep in mind that the stronger the computer is, the less lag it will go through, and vice-versa.
Factors that Affect Lag
- Operating system. (Windows 8.x worsens performance, if not patched, whereas Windows 7 is optimal. Windows 10 now has a patch, but it still worsens performance without it, just like Windows 8.x.)
- SoundFont. (larger soundfonts are more bearing on the system's resources)
- Synth. (or lack of synth; different synths are better at handling MIDIs)
- CPU: single-core performance. (Intel Haswell CPUs have much higher performance in MIDIs than older microarchitertures.)
- GPU: Graphics performance for Synthesia and MIDITrail.
- Computer load at the time. (what programs were open, etc.)