Actually there are many reasons that make the cars slower in the first lap of a race.
- Cars accelerate from 0 kph
- Traffic is at maximum
- Tyres are not at ideal temperature (still soft tyres can perform at peak)
- Fuel tanks are full
On the other hand, the first lap is shorter than other laps on most circuits, because the start line is usually ahead of the finish line. In addition, if it starts raining in the 1st or 2nd lap, the fastest lap can be easily set in the first lap.
I'm questioning whether this lap can be officially recorded as the fastest lap. Since the distance is different from other laps and also different for each car, I would expect the first lap to be excluded from fastest lap records.
I checked the 2019 Formula One Sporting Regulations, but couldn't find anything relevant. The following part in Article 6.4 is about the 1-point award for the fastest lap, again nothing relevant.
In addition to the above, one point will be awarded to the driver who achieved the fastest lap of the race and to the constructor whose car he was driving, provided the lap time was achieved without incurring a penalty and he was in the top ten positions of the final race classification (see Article 45). No point will be awarded if the fastest lap is achieved by a driver who was classified outside the top ten positions.