The Midibox sequencer's response time is measured in nanoseconds. Of course, as you point out, MIDI is a serial protocol and very low bandwidth, so there is a limit to the timing accuracy of supposedly simultaneous events on a single port*. If you are using a multi-port MIDI adapter, it should in theory be possible to have much tighter timing between events on different ports.

*NB: not MIDI channel. There are 16 logical MIDI channels per physical MIDI port/bitstream.

I expect that a Classic should have little trouble being as timing-accurate as a single MIDI port is capable of.