My back tire is rubbing on the frame and is having a great impact on the speed of my cycle.
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Your wheel axle is not aligned in the dropouts properly, causing the rim of the wheel to deviate to one side and contact the frame.
I'm guessing you have horizontal or diagonal slotted dropouts and the drive side of the hub has been pulled forward by the force of the chain. You need to re-align the wheel.
This video from Park Tool is about single speed bikes but the wheel alignment art applies to derailleur equipped bikes as well.