At WDW I have never had to explain what my medical stuff is or even that it is medical stuff (though it should be pretty obvious it is). YMMV. (at airport security I have to explain -- keep it short and simple: e.g. "medical liquids for my nebulizer"; if they need more info they can ask more questions and I will answer.)
I long ago got over holding up lines. I take the time needed and if the people behind me don't like it, that is their problem, NOT mine.
For example, it takes me ~10-20 minutes to get through airport security (not counting line wait time) because I usually will set off the metal detector and have to be patted down + they have to swab my medical liquids and sometimes have to search a bag to find something they think looks weird (e.g. last time it was to find the four
AAA batteries I had together in a plastic carrier (?!?) ). The people who were behind me are long done and gone by the time I get to leave security.
At WDW, I prepare for the bag check by packing my bag(s) in a way that facilitates the check. That includes the clear bag(s) to protect and make checking the medical things easier. Likewise, instead of having lots of little loose things, I make use of small bags to aggregate things -- things are less likely to fall out and go missing, and there is no pawing through all the stuff. Lots of people don't prepare at all, and quite frankly they usually take longer than I do to get through.
SW