execution time increases after turning off read replica in postgres

The task was assigned to me to check the performance of uploads and query execution separately for "read replica" turned *on* and turned *off*. Surprisingly when "read replica" was turned *on* the performance was better. Note that in both tests, made sure that DB is freshly refreshed. Coming to HA in Postgres, I have verified that the process of archiving WAL logs is happening in DB irrespective of read replica enabled or disabled(I feel it is obvious based on the parameter). Now my question is how is enabling read replica improved the performance of DB activities as a whole. Also, *checkpoint* switching seems similar for both tests, can you put some light on this, please?