I am having similar issues and can report a bug to apple if needed.
I purchased and installed on Wednesday and was able to play without issue until I reached the middle of Cyseal city. At that point the lag became so terrible I couldn't play it. I looked up solutions on forums and implemented what I could. All setting changes and macbook cleanup made little difference.
At that time I was running Yosemite. After trying everything I could for the next few days I upgraded to El Capitan. That made no improvement to the lag and in fact introduced a bug that crashed the game every 10 minutes. Went to the forums again and nothing I did seemed to help.
