I can speak from personal experience on this. Last fall, for a period of a few months, I was constantly exhausted when I would arrive home from work. I've never been a high energy person but this was very, very unusual. I'm talking get home and barely have the strength to do anything other than watch TV. This killed motivation to practice as well as the ability to practice. When I DID practice, I was unable to be as sharp mentally as needed. I had no symptoms other than the energy.
After Thanksgiving, I was hospitalized and ended up having surgery to repair some issues. Turns out those issues had been causing me to be ill for months and I didn't know it. After a short recovery time, I suddenly had energy again and could actually practice. Since December, I've lowered pretty much all of my PBs in every event - except clock, LOL. And not just PBs but overall performance - I went from having just a few sub-20 3x3 singles ever to getting 25 of them since February 1st.
So, yeah, health can definitely have an effect. Especially if you don't even know you're sick.