Personally I would never have a bitch spayed if i was racing her................ it does take away their pray drive, hence speed, why have them done if you don't need to.????? just my opinion.
http://www.delano.k12.mn.us/high-school/academic-departments/science/mr-b-wiesner/cross-country/10-things-you-should-know-about-lactic-acid
Take a read.
I think it depends on your daily routine or how hard you train or work them, as long as you don't feed straight before exercise you can't really go to wrong not everyone's the same. good luck with your pup.