I've been there for two years during the taping. I think it is usually somewhat less busy on that day in regards to riding and eating, etc, since a lot of people are basically sitting/standing on the parade route during filming--which lasts ALL day btw. (they film each float/dance/song at least three times so it is long and monotonous! They spend a lot of time saying "smile everybody, clap--look excited, look happy" when you are mostly bored out of your mind watching them film for the third time--and it is a lot of waiting in between takes too)
So, it is "busy" or crowded on main street and around the hub while they are filming there, but the rest of the park is "usual" or "normal" busy--which is to say, not so busy since it is during a non-peak weekend.
TBH, the parade filming is a bit of a nuisance since they take up main street and it can be a bit hard to navigate through that area if you need to, but it is also kind of exciting I guess, if you want to sit there all day in the hopes of getting on camera, or to see the celebrities they bring in for the parade. We always do a tiny bit of both--sit for a few parade shots, watch a dance/song number or two to see the celebrities, then move on and enjoy the park.
No worries, you will have a fine time on that day if you don't waste your day, I mean, spend your whole day, LOL, watching the parade.
Oh, one last thing--keep your eyes open all around WDW during the parade weekend for the various celebrities and their families enjoying the parks and resorts. Don't bother them, of course, lol, but they are often riding the rides and enjoying the pools while the "working" member of the family is taping the parade, rehearsing, etc.