You do not need to save rebecca from the hunter by the serum room!!

All that is required to save rebecca is to enter the serum corridor from the first floor and leave, this saves time on killing the hunter and the cut scene which follows, however you need to bank somewhere which means banking under the stairs where the chemical was. There is also a First Aid Spray here if needed. It is definitely faster than fighting the hunter and watching the cut scene but not sure by how much since I am relatively new and can't time it with optimal movement.