Why does she take so long to reply?

We used to to best friends growing up as kids. But she moved and we don't talk at all really but still stay in touch. Well we both have iPhones, and you can see when someone has read your message or not. Well she sent me "hey" and I didn't get it until a few hours later. I replied with "hello" and it took her a day to reply back. Now I sent "what's up?" and of course I didn't see when she replied the second time. It's like she's trying to match how long it takes for me to reply. The difference is, she sees the message right away and I don't.

What is so hard about replying sooner? And it seems like only girls do this. Why?

She found a new best friend OR lets face it kid, this is how girls work: If you take forever to reply they will reply with the same amount of time you replied. & if she saw your text she wants you to know how it feels for it to take a while for you to reply.

Maybe she's busy dude. Chill out. Unless you're asking her a really important question, she probably sees no reason to reply immediately, and then forgets. It's not only girls that do this, I've gotten plenty of texts and messages that I've taken awhile to respond to, that's the entire point of a text message, they're convenient because you don't have to reply RIGHT AWAY. If you have something urgent, use the phone.