Which version are you referring to? The latest stable or testing? In the current testing versions, waveform drawing is all done in a backbuffer to prevent update artefacts when desktop compositing is enabled. While the sample is being drawn to the backbuffer, no GUI updates will happen so yes, it will look like OpenMPT doesn't do anything. However, if anything, it should make the whole process faster rather than slower, so OpenMPT shouldn't be as unresponsive as it used to be.