The delta is a bit more nuanced than that. The drag coefficient for a delta wing only impedes flight at low speeds-low altitude flight profile, but does better than other wing types in high speed, high altitude, high maneuver flight profiles.
Once the plane catches speed, there is a change in how the fluid dynamics work in favor of the delta. Check this :
https://en.wikibooks.org/wiki/Fluid_Mechanics_Applications/A02:_Delta_Wing_Aircraft_Aerodynamics
The primary advantage of the delta wing is that, with a large enough angle of rearward sweep, the wing’s leading edge will not contact the shock wave boundary formed at the nose of the fuselage as the speed of the aircraft approaches and exceeds transonic to supersonic speed. The rearward sweep angle vastly lowers the airspeed normal to the leading edge of the wing, thereby allowing the aircraft to fly at high subsonic, transonic, or supersonic speed, while the over wing speed of the lifting air is kept to less than the speed of sound. Some other advantages of Delta wings are:-
- Rise in Transsonic drag is gradual and peak drag in supersonic flight is reduced
--Lift is spread over the broader chord
--Senstivity of Drag to Mach Number is redued greatly
--Satisfactory cross sectional area can be achieved very easily
- Change in lift component CL and lift curve slope CLα with the value of Mach Number M is gradual.
- A very high stall angle is achieved by increasing the angle of atack, as a flow energising vortex is generated by the leading edge of the wing at high angles of attack.
- It allows for light wings with high bending and torsional stiffness.
--More volume for fuel and gear is available in thicker wings.
--Fluttering and aileron reversal problems are wholly eliminated.
- Acceptable level of maneuverability and ease of handlingis achieved because of low wing loading.
- Wingspan is small and hence does not require folding.
- Available area for external storage is large enough.
- It is easy ,simple and relatively inexpensive to manufacture.
- The addition of canards to the aircraft increases the overall lift,hence enabling the aircraft to perform extreme maneuvers and at the same time improving the low speed handling and lowering the landing speed
Couple that with relaxed static stability and you have a plane that is probably the most suitable plane for emerging victorious in dog fights.