Geoff: On a DIN MIDI cable, the speed is 31250 serial bits per second and each byte of data is made of 1 start bit, 8 data bits, and 1 stop bit. Since there are 10 serial bits per data byte, that works out to 3125 data bytes per second. (Source: The MIDI 1.0 spec
PDF page 6 (printed page 1) section "Hardware" begins "The hardware MIDI interface operates at 31.25  Kbaud,  with a start bit, 8 data bits , and a stop bit. This makes a total of 10 bits for a period of 320 microseconds per serial byte." 320 microseconds per byte or 0.00032 seconds per byte is the same as 3125 bytes per second.)
Dolores: As you mentioned, on a DIN MIDI cable, MIDI messages can be sent at a maximum speed of 3125 bytes per second. In the situation you describe (notes are sent on seperate channels and each note is prefixed with a Pitch Bend), and in the worst case where Running Status is never be used, each note will require:
3 bytes for the Pitch Bend message
3 bytes for the note starting message (Note On with Velocity over zero)
3 bytes for the note ending message (Note Off or Note On with Velocity zero)
So that's 347 2/9 notes per second.
If the connection between the keyboard and the computer is a single USB cable (from a USB port on the keyboard to a USB port on the computer), then my understanding is a USB MIDI connection uses a faster speed, but I don't know the details. For example, page 10 of the USB MIDI 1.0 spec
mentions "The USB transfers MIDI data at rate hundreds of times faster than the original MIDI 1.0 hardware specification."
I'm not sure what to suggest next. Can you give an example of the problem you are having? (For example: What keyboard are you using? How is it connected to the computer? What symptoms are you seeing that suggest some kind of buffer overload?) Maybe we can think of some kind of test to help tell where a buffer problem might be located. (A buffer in the keyboard? A buffer in the MIDI interface? A buffer in the MIDI driver or operating system? A buffer in the DAW software?) Maybe someone else will recognize the kind of problem you are having and have a suggestion that might help.