... If you have a lot of bands on your account, it can slow down the processing time to get into the parks at the turnstiles.
I really have to ask: who did you hear this piece of blatherskite from? Was it a
real WDW bus driver? or one of those fake ones masquerading as a Park Entry CM?
A Magic Band is merely a key that lets you access your account. It's a 1:1 (one to one) relationship going this way. One band sees one account. Not only does the system not need to look through your entire collection of bands to let you in the park, it can't possibly even know that there ARE other bands (keys) associated with your account at the point of entry. As messed up as Disney IT is for most things, even they would not want to do that (well, they could do it if they really, really, really tried, but they may not be that smart and to what possible end?).
Besides, deactivating old MBs doesn't do anything except prevent them from actually performing any transaction. They're still associated with your account, they still show up in
MDE, and they can be re-activated at any time.
Now, are there other reasons to call Disney IT and get them to completely remove old stuff from your account? Sure. There are many, many reports of MDE accounts getting so overwhelmed with all the interconnected bits of data from every trip made since the inception of MyMagic+ that they completely crash and burn, never to function again. But the Magic Bands themselves since they have 1 and only 1 function - to be a key, aren't even close to being the cause of those problems.
Steve