Hi @Dithered42! From a recent post in another thread you demonstrated using the stable release. That has a very different version of MIDI recorder / player which did suffer from long delays. The new version is code written specifically for Zynthian - bespoke code which does a lot to optimise these elements. However, despite the data being (apparently) thinner, it can still be time consuming to read a large, complex standard MIDI file (SMF) which can lead to some delays with such files.
The next release of Zynthian is due soon. You could wait for it or even get involved in testing it by switching to it.