rsmb

I have an i7 8700k 4.3 ghz 16gb tridentz rgb 3200 mhz ram 2070 rtx geforce graphics card. I render at 2560x1440p and it takes a 30-40 minutes video 50 ish minutes but when i render a 30 seconds clip with rsmb it take literally 30 minutes. I dont understand why. Let me know if you have any...