I think this one comes under the "everybody's different" banner. I have not ever suffered a decrease in performance because of my period. In fact I think my endurance performance has been improved.
Why? Not sure if it's because of some perverse attempt to prove that it makes no difference and I actually end up trying a bit harder, or whether those early years of period pain conditioned me to suffer through more at that time, or whether being on the pill has damped down any ill effects.
Whatever it is - I'm happy with the outcome. As long as I can handle the logistics of athletic endeavours with periods (and there are plenty of threads on that) I'm good to go.
You know your body best - I figure there are always going to be days when we feel flat for whatever reason. This just might be yours. But you may well be fine too. I guess you have to ask yourself... "Would I drop out because I am going to feel flat - for whatever reason?" Perhaps make sure you get a bit of extra sleep and some extra carbs (or whatever your body may crave at that time - listen to it - sometimes it seems nonsensical but it knows what it needs) and just get in there and give it your best on the day.
I always find that the more you worry about it now and the more you're looking for it on the day, the more likely you are to find it! Just enjoy your ride.![]()