PS2_MOUSE_ENABLE ridiculously slows down matrix scan rate

[tomykaira]

Describe the Bug

PS2_MOUSE_ENABLE with PS2_USE_INT ridiculously slows down other procedures such as matrix scan.
With rough testing, it seems like matrix scan counts per second decreased to about 1/10.

This is because ps2_host_send() in ps2_interrupt.c uses lots of delay().
Inserting return to the first line of ps2_host_send() solves the problem, but then PS2 mouse does not work, of course.

System Information

• Keyboard: my own hand made keyboard with X19 trackball
• Operating system: macOS
• AVR GCC version: 8.2.0
• QMK Firmware version: Fork from tag: 0.6.302
• Any keyboard related software installed? : no

Additional Context

As far as my knowledge goes, the only way to solve the problem is to rewrite ps2_host_send() using timer interrupt instead of delay()s.
I will try it, but it seems hard.

[alex-ong]

Timer interrupt may not be necessary - we could potentially use a time-delta method and call ps2_host_send() every scan?