Its not just that the bus service from Cheadle to Manchester is poor is the route and amount of time it takes to get to Manchester as it doesn't take the direct route instead it runs via Didsbury, Withington, Fallowfield and up Wilmslow/Oxford Roads which are known for generating traffic especially through Curry Mile through Rusholme
This is a good point. There have actually been faster services in the relatively recent past.
Stagecoach tried the X57, which was the same route as what is now the 42B, but with limited stops north of East Didsbury. But this only lasted a few years, and in any case it was still ridiculously slow at rush hour because of traffic.
Then there was the long-standing route 130 which ran via the A34 rather than Wilmslow Road, but that service suffered a death by a thousand cuts at the hands of Arriva, and the section between Heald Green and Manchester is now gone entirely.