The heavy part is not the MIDI, but the virtual instruments here. The Buffer is not about the CPU only, it’s about the whole communication among hard disk, RAM and CPU.
That doesn’t make sense to me. It seems all it has to do is to send the MIDI info to the VSTi’s at a different rate.
However, I suppose if there are any instruments or effect that might be set to the tempo for timing that might have an impact. That would be a very badly behaving plugin.
Anyway, originally my buffer was at 192. I put it to the max of 2048 and Cubase still choked on an automated tempo change (tempo track) in exactly the same way.