interference, distance, more clients on AP, client chose to remain connected to weaker AP rather than roam to stronger signal (they do that and it's the cleints decision), connected initial data rate unsustainable so dropped to sustainable rate...probably other reasons but that'll do for a start.
Be pleased you are connecting at 140!
A solid reliable connection is far more important than maximising throughput.
After a reread I might be missing your intention. Please provide more info about the speed drop, ie what device or widget is reporting it.