Under what circumstances is she moving slower (in a pasture, under saddle, at which gaits), and what is her current condition (age, pregnant or not, exercised regularly or sedentary, sound or lame)?  Without more specific information, it's difficult to even begin to speculate.

Tempest

It's nice of you to be concerned about your neighbor's horse, but there simply isn't enough information in your posts for me to have any idea what might be going on with the animal.  It could be nothing at all, or it could be something a vet should look at.

Have you considered voicing your concerns to the horse's owners?  Let them know that you have noticed a significant change in the horse's behavior and activity level lately, and you're worried that there might be something wrong.  The owners may not even be aware that the horse is acting differently, so your comments may prompt them to pay closer attention to her.

I'm sorry that I can't offer any more specific advice than that.

Tempest