I think that you're going a bit too far with 'far' faster - there isn't any evidence that this is the case and a lot of engineering effort has gone into making Arm fast - not least from Apple.

Apple aren't moving to Arm for no reason though so I think it's worth considering the reasons why this might be the case: power efficiency (probably the most important), price, ability to incorporate custom silicon, a single ISA to support, full control over the stack, access to best manufacturing process.

On the last point, I think Apple (Tim Cook especially) has probably lost faith and patience with a company that has overpromised and underdelivered for a number of years.