In "Preface to Shakespeare", Samuel Johnson wrote
It may be observed, that in many of his plays the latter part is evidently neglected. When he found himself near the end of his work, and, in view of his reward, he shortened the labour to snatch the profit. He therefore remits his efforts where he should most vigorously exert them, and his catastrophe is improbably produced or imperfectly represented.
Othello is no exception, until the fourth act plot goes smoothly, however, in fourth and fifth acts everything tumbles down unnaturally.