Head over to www.propellerheads.se and download version 3.0.5 This is the MAC Universal compatibility update.

On a Macbook Pro, all older non-Intel based mac apps are run through an app called Rosetta, because of this you will get hampered performance from processor intensive apps such as graphic and audio apps.

The 3.0.5 update allows you to run Reason as an Intel Mac native application, and should solve your lag problems.

Hope this helps.