I have experienced similar on rare occasions, although speed picked up again 20 seconds after letting it run slow. Never had again since 1.4.
Shot in the dark... But try to type these three commands into the bottom right max script command window.
gc() Garbage Collection.
freescenebitmaps() Cleans up in texture memory.
clearUndoBuffer() Removes your Undo's
Logic being, that in heavy scenes all Undo steps are saved and can add up to gigabytes with heavy changes respecrivly. We may crash past our max's ram limit and it started paging out parts of max to compesate, which is a classic ruiner of performance.
If this is the case, then undo delete won't immidietly pickup the speed again, but maybe 10 secs later.