I've had the same problem, it was smooth for previous updates but started lagging heavily since the new update, it's basically become unplayable
Can you try r3.2 and tell me if it lags? I want to see if it's still lagging if you go back to r3 after it starts lagging in r4. (https://mega.nz/#!icUTVCYQ!KLgm9uhYxFBzZ6AZT1Z7baqQh-QF_CshiOm6nDEc_i0)