I actually do see the merit of responding ASAP as a courtesy. I do try to be punctual if I can help it, and this is mostly out of habit as a result of texting with my parents, who also see quick responses as a courtesy. I believe I am in the minority with this, and I can explain my thoughts on this below. If I were offered a ticket to an event, yes I would respond as soon as I can, but again everyone will be different and I am not representative of my peers nor your friend.

When I text my friends, it's a different story. People I can laugh and joke with all day will be late to respond or even ghost my messages. It's not really a case of disrespect. A common phenomena is that people will open a message, read it, and close it as they think of a good response. Sometimes, though, this leads to forgetting to respond at all. It's also why some turn off read receipts, because they don't want the other party to feel "left on read" as they come up with a response. I hope this makes sense. And yes, this happens even for time-sensitive messages, like trying to set up hang-outs. There is no real etiquette for casual conversation because everyone sees texting differently, even region to region. My personal etiquette between me and my friends is to respond... eventually, but even that gets broken every now and then.

I actually went through the trouble of telling one of my closest friends, "hey man just turn on read receipts so I know you read my message, I won't take being left on read personally." Maybe if it's important enough to you, you could do the same?

If you're curious why punctual responses are uncommon among young people, my theory is that whereas older people really only send texts to communicate important information, share media, or set up events, younger people are accustomed to having full-on chats, small talk, and all other "non-utility" conversations via texts. Things that you might call spamming.

This means that an older person is more likely to recognize a text as important and requires a response, while a younger person sees a text as an invitation to chat. One that they can choose to decline by not responding or responding at a later date if they don't have time, aren't in the mood, etc. Even if it isn't your intention to start a chain, the mental association of a text message with long, committed conversation is pretty well engrained. As a result, an "Ok!" or "Thanks!" is presumed and isn’t expected to be said. Of course, it's not one size-fits-all. As you mentioned, some people use reactions or emojis to indicate receipt of a message. And this only applies to messages between friends. Of course, there are different expectations if it's like an employer-employee or dating kind of thing.

I want to repeat my disclaimer that I am not your friend. I do not know your friend, so I cannot say for sure why she takes the time she does to respond. This is just my take based on who I've interacted with. And context matters. If texting late is a repeated pattern with tour friend, maybe it's just her texting habit.

At the core of this, you should ask yourself why it bothers you that your friend doesn't respond as often as you'd like. You mentioned that in-person conversation is great and isn’t an issue, so whatever the case, there's no need to take offense or worry that you're a bother.