Does sex get better over time with SB's? I think it depends on a lot of factors. Most of the time there's anxiety at the beginning. It takes time to get comfortable and trusting with someone as a sexual partner for both men and women, but especially women. Once trust is established and a comfort level sets in, it's easier to let go of inhibitions.  

For me it seems to get better as time goes on, but often takes 5-6 sessions for the walls to drop enough to feel comfortable. I've had bad experiences with both escorts and in the sugaring world with dishonest partners and mentally unstable girls so my spidey senses are always on alert at the beginning.  

As far as sexual performance goes, I've found a lot of non-escort women just don't have much experience pleasing men. Some girls have only been with a small number handful of partners. A few of my SB's were only experienced with one or two partners, and those partners never communicated to them what pleased them.  

I like showing a partner how I like certain things, depth, speed, sounds to make and so forth. If these 'trainings' are done slowly and with care she will integrate them into future lovemaking sessions and it will please both of you more since she will likely respond with more arousal if she sees you are liking what she is doing.

On the other hand, some girls are just not comfortable sexually and no amount of education will get them to be responsive, [aka 'the starfish response']. If multiple educational sessions do not yield adequate results it's time to move on, but don't give up prematurely as she may just be showing signs of anxiety and trust issues, or just not have the experience.  

Sometimes still waters run deep.

I do.  I expect her to get more comfortable with me.  Meaning more open to long blow jobs.  Wearing lingerie for me.  Sending nudes.  Toys.  

If she gets lazy because we are comfortable, then it's boring.  Some people are ok with that because their partner got your body wired.  So she knows the BJ you like and how to get you off.  I had an ex who loved me being lazy in bed because I know the two or three things that made her cum.  She was completely satisfied.

I've been seeing this gorgeous Malaysian lady for a little over 2 years.  She was pretty vanilla but about 6 months ago I  bought her a vibrator and butt plugs. Turns out she's freaky. Even begs me for anal. Pleasant surprise.

If the communication gets better, which it definitely should, the sex will be better.  And it shouldn't take years.

I agree with Herb on this, in general. With time you can get more comfortable and learn how to please each other and the sex is an expression of your feelings for the other person.
Of course this depends on both parties really liking each other and wanting that relationship. Otherwise it can be a chore.