With any group of supposedly identical slot cars there is bound to be a spread in performance, the difference between the slowest and fastest cars is likely to be significant. The best that you can hope for is to have enough cars that are reasonably equal in performance. You might take a look at this tuning article:
https://drive.google.com/file/d/11tPoKL ... sp=sharing .
You could work on the slower cars and improve their performance for a start. If you intend for the cars to be used as loaners in a formal race of some sort it is much easier to have a car in each lane that stays in that lane while the drivers rotate. That way the only difference would be the drivers themselves because everyone would be driving the same set of cars and their individual performance would factor out.
With pancake cars one reason for the variation in performance is likely to be the armatures, those can vary by a considerable amount. Replacement "Mean Green" armatures for A/FX cars are still easy to find and you could also use Johnny Lightning/Auto World armatures. You could also narrow the performance gap by putting slip-on silicone tires on the slower cars and leaving the stock rubber tires on the faster ones.