and that the speed of the Lancaster could be increased by 50 mph by dispensing with guns.
As the speed of attacking fighters would almost certainly be 50 knots or more than the cruising speed of a Lancaster (especially in a dive quarter attack)then the extra speed of the Lancaster without armament would be useless.
In any case knowing a bomber was unarmed would encourage the enemy fighter to get in real close before firing. From vague memory when I flew Lincolns in 1953 with a 20mm dorsal turret and two 0.5's in the front and rear turret, there was little significant IAS differences from other Lincolns I flew with no turret and other guns removed.