Been through Singapore's Terminal 1 a few times this year at peak periods and never less than 45 mins wait at Immigration both inward and outward. Last time had a 5 hour transit again at SIN T1 and the eateries upstairs were a disgrace with filthy uncleared tables. The security staff at the gate lounges can be very abrupt too. Yet they consistently win awards for world's for best airport !!
From my experience MAN when busy is no worse, and a breeze at off peak times.
Are you honestly comparing a 3 terminal airport in MAN which handles around 22 million to a 3 terminal airport in SIN which handles 54 million passengers.
Compare like with like and you will soon see MAN worse than similar airports of its size.