Delay with MIDI only use

I am frustrated trying to use Cubase LE for MIDI only recording using an external MIDI device. The first track (single note melody) records and plays back as I would expect, but as soon as I overdub a second track there is an erratic delay on the monitoring of both the previously recorded track and the new track I’m playing live. It is basically impossible to play sensibly like this. What is causing this? I am not using any of the computer’s audio except the metronome. Years ago when I was using Cubase Score and Windows 3.1 a much lesser spec computer was able to handle the MIDI signal instantaneously. Has technology gone backwards?

Grateful for advice.

Cubase LE
Windows 7 operating system
Internal soundcard: Realtek High Definition Audio
MIDI interface: Midisport USB 2x2