In the UK it is compulsory to run front and rear lights for all time trials (there are no rules on the size or brightness though).

Aerocoach actually did some testing and found a "sensible" front light made pretty much no difference to your overall aerodynamics, we're talking less than 2 watts at 45km/hr (28mph).

https://www.aero-coach.co.uk/front-light-aerodynamics