It seems reasonable to me to compare the times of two horses that ran at the same track, over the same distance on the same day (especially for sprint distances).

In fact, assuming that:

1. The condition of the track did not vary (i.e. become wetter or dry out);

2. The tempo of the races was similar; and

3. there were no other factors, such as a headwind, to influence the times;

then the only sensible analysis is that the quicker horse should receive the higher rating.