poor performance when the measure is very large

MuseScore currently lays out the entire score after every edit. Work is being done so that with a future version of MuseScore, when you make a change, only the page(s) on which the change is made will be re-rendered, and the rest of the score will be left alone, which should solve this problem. Watch for a post about this in the coming days.

I'm attaching the file here for future reference in case the musescore.com link changes.

Meanwhile there are a few things you can do to help. One is not use the Navigator - that is basically asking MsueScore to lay out the score *twice* on every update. Another is not generate linked parts until you are finished editing - parts also basically doubles the work of layout. I think also Continuousview tends to be less efficient than page, although there might be other variables involved there.

Consider breaking large scores into separate pieces while working on them and using the Album feature to combine then when done.