low water pressure in house caused by hot water tank coming off main before cold?
Unless the size of the pipe was reduced at the tee The pressure/flow should not matter where the water goes into the heater. With that said I have had gate valves fail and not open all the way after being closed for maintenance I would suspect the main cutoff valve to be the issue since copper dosent corrode and shrink down like galvanized pipe will over time. The last possibility is when the work was done some debris may have broken loose and partially plugged your valves if washerless or faucet ariators. I would be checking these items before paying for another call to the plumber.