Lack of awareness or arrogance.

I wouldn't say arrogance is the right term.
More like a complete lack of awareness, combined with a sense of entitlement.
She sees herself as the centre of everything and everyone else revolves around her.
The lack of awareness is-
Because she has plenty of time, she fails to grasp that other people may be in a hurry, so doesn't care about holding them up.
Also,
(ties in with lack of awareness plus a bit of stupidity?)
She only has to remember ONE Recepionist, so talks to the receptionist as if she's a long lost friend, but fails to grasp the fact that the receptionist deals with HUNDREDS of patients so doesn't know her from Adam and doesn't want or need to know about her private life.
My late mother in law was like it.
She would get quite upset if a Doctors receptionist was 'frosty' and didn't remember any of her previous gossip.
Trying to explain to mil that she was one of many in the receptionist's eyes would fall on deaf ears!
Please God shoot me if I ever get like this

I was very embarrassed when trying to sort out where a text had come from ( nhs website) but the receptionist didn’t have it on her system.
I just kept apologising to everyone behind me.

How lucky you are in being able to physically enter your surgery. The door to our village surgery is still locked, prescriptions to be posted through the letter box, telephone consultations only except under extreme circumstances and everything conducted via the receptionist. My previous doctor retired pre-covid, and I have yet to speak to her replacement despite several attempts; I wouldn't know her if I fell over her.
Because I live in a village many customers in the shops are known to the staff, and the lengthy conversations take place regularly; I am generally not in a hurry so it doesn't bother me unduly; when I was working it used to infuriate me.

Sounds just like a friend of mine. I really wish she wasn't a friend but she has been since school.

I despair when she happily tells me she will make a GP appointment last half an hour. She gets up to leave, then sits down again. As she's leaving something else comes to mind.

I've told her that is hugely selfish but she just thinks I'm babbling on about nothing.

Both! The same applies to people standing right across the pavement whilst having a conversation, seeming oblivious to anyone walking on the pavement.

Is it lack of awareness arrogance or selfishness or own importance or loneliness I wonder. My biggest annoyance is customers and staff at the tills constantly talking holding people up.
I had an annoying experience at the Drs yesterday not someone gossiping but unorganised chaos. I was sent a text message the form was on reception for my blood tests. I was Just expecting to be handed it I was kept waiting this was 11.40. The form wasn’t ready to go upstairs for bloods I had to explain I made an appointment online of which there was no record on her computer.
The phone rang and a patient came in know one else was there the place was empty! I was told the receptionist had to print my form out I got upstairs for bloods just in time at 12.00 for my tests. By then my blood pressure alone was on boiling point.

A combination of both? Her need to monopolise people is more important than yours, in her mind. Or what she is saying is much more interesting and therefore it gives her the right to keep blethering.
Or she might be unaware, suffering from dementia or very lonely.
I would grit my teeth and be resigned to the wait.

I have come across this in my GP surgery, the pained expression on the receptionist face says it all. A lady had been in to see the Dr, then come out to the receptionist to ask her to fasten her coat up, then proceeded to regale all and sundry about what the Dr had said. It’s like people on their mobile phones, telling a bus load of people what their best friend has been up to. Totally self absorbed is what I call it, as if anyone else wants to listen to their woes.

Probably both.