“It’s a very old Yorkshire name. Once it took the head of my family a day’s hard riding to make the circuit of his estates, but the mighty are fallen. Fast women and slow horses.”

The meaning here is that the family has lost all its wealth for the following mildly humorous reasons.

Fast women This a rather old-fashioned expression. A ‘fast woman’ is a rather immoral woman who likes a man to spend all his money on her.

slow horses A slow horse in a horse race is not a good one to bet your money on.

That phrase means ‘he became bankrupt’

That phrase wear in movie Sherloc Holmes(2009)’s dialogue too.

Watson: Lady Radford’s emerald bracelet has disappeard.
Holmes: Insurance swidle. Lord Radford likes fast woman and slow ponies. (He become bankrupt)

Fast Women are women that slow men cant keep up with better watch yourself because women are moving quick today.

You want your women to take it slow and your horses fast. The opposite, fast women, are thrill seekers and easy. Slow horses will ruin you because you bet on them and they lose.

comparing someone’s intelligence to the speed of a horse finishing a race. the slow horse would naturally finish last, therefore the person who is the ‘slow horse‘ is not very bright.