B737-800 Captain reported a flight control system malfunction and hydraulics system quantity decreasing made flight control sluggish during cruise. The crew decided to return to the departure airport.

Narrative

During cruise flight at FL350; FLT CTRL SYS B became inoperative. We completed QRH procedures which directed us to place the FLT CTRL B switch to RUD STBY. This also created additional failures in Diff Feel and Yaw Damper Fail. We also noticed that 'A' system hydraulic fluid appeared to be decreasing. Since we were not yet halfway and about to go on a portion of our trip completely over water for approximately 500 miles; we elected to turn around to divert back to ZZZ. We considered diverting 300 miles to ZZZ1 and looked at other potential diversion points on the return to ZZZ. We were going to be approximately 2;000 pounds overweight; so we [requested priority handling] in accordance with the FOM and company policy. During the approach the flight controls were sluggish. The approach and landing were completed per the 737 QRH Overweight Landing Checklist; and an overweight landing entry made in addition to the SYS B Failure in the a/c logbook.Suggestions - I think the 737 QRH needs to be reviewed for additional steps on this item. Very brief. Doesn't mention that follow on failures will occur including loss of Yaw Damper and Feel Diff. Additionally there is no information in QRH Overweight Landing Checklist that indicates[requesting priority handling] is required per company policy.