When a provider comes to you and is going into the bathroom or when she arrives say you would rather she not change into lingerie . Just come out in her birthday suit. Problem solved.
No need to add the additional commentary you wrote about mechanics...attys..etc many of us get it.
I have some gents who feel like you and I come out nude and others who appreciate the lingerie or specifically request it even if not being worn for very long.
Just be upfront .

You deserve to receive every minute of the time you are paying for. Spending the first few minutes in the bathroom is unexceptable.

Bud, just say so? So, so many ladies want you to have a great time.

Most ladies will hate me for saying it but I come with my lingerie under my clothes because I like to make out and undress each other. I am not interested in some magic trick where I disappear dressed and reappear in costume. Unless of course he requests that I come out in some sort of costume. I like to undress slowly in front of my date with his hands all over me.  

Couldn't agree more. I have had this done to me by many ladies, including ladies who have replied on this thread.  They take 15-20 minutes in the beginning, and another 15-20 at the end.  It makes it awkward to ask "everything okay in there?" after 10+ minutes of waiting, especially more than once.  It kills the mood and detracts from the value of the service which is why I do not repeat with providers who run this. To each their own; luckily Vegas has a lot of options for high end providers who aren't just trying to run out the clock.

Not just Outcalls

I am a long time local and this has happened to me several times in a lady's incall also. Not just the bathroom routine, but over use of cell phones during. Once I was at a high end (for me) apartment and the lady said we have to wait for her grocery delivery!
Any West Side ladies with incall who know how to make 60 or 90 minutes a true fun time? No trick add $ 200 for a hotel room either.

Fortunately, I thoroughly enjoyed a couple of providers who were totally cool with my request this past weekend and provided perfect GFE outcalls. Although there were quite a few others who failed to respond whatsoever when I contacted them via email or text. So I stepped up in class and went with the highly reviewed Emi Rei who met and exceeded all of my needs and desires! In the past, it seemed like I could narrow down my search to two or three providers before my visit to Vegas. But now I need to expand my list quite a bit to account for the higher number of non-responses than in the past. I think there were several different conventions in town this past weekend so hopefully most of the providers were just busy with business to respond to all of their requests.

She hates her job and was just killing time or doing more than changing clothese.  

This is a reason we have the review system. Post a review,  When too many recviews show up with this pattern of "20 minutes to change" - guys will move on and selecdt somebody else.  

To the ladies who have shared they are already wearing something more comfortable under street clothese - clever!

but my advice is to not book with that or those ladies again. They’re running out the clock so they don’t have to put in as much work. It’s sad that this type of attitude is so prevalent.

It blows my mind how many girls think the biz is about looking perfect. The truth is,  most guys ( young and old) have said to me that they would rather have an hour with someone like me who truly enjoys the experience. If they want only looks they would go to a strip club and watch  the centerfold!