freeze MIDI modifiers -- taking too long

could anyone explain why the freeze MIDI modifiers command takes such a long time to complete?

even when processing a single MIDI track, using no MIDI insert plugins, containing but a single 1-bar MIDI part with a couple of block chords; basically the only post alteration to the part being transposition via the infoline.

now, to ‘flatten’ the part, cubase takes 6 (!) seconds. more than actually bouncing it to audio.

what is happening there that makes this so unbearably slow?