I would say that yes, it’s likely normal considering you pointed out that even though she is fluent in English, she isn’t well written. Perhaps she realizes this about herself and texting is a source of insecurity for her. If that is the case, I can assure you that she is spending more time analyzing her OWN behavior and response than she is thinking about how much time has passed since she responded. Secondly, how do you handle it? You don’t. Be patient with her. She’ll respond when she can. If you feel texting is really getting in the way of healthy communication, you do have her phone number and she indeed has a phone – pick it up and call her. A lot easier and much less confusing for everyone involved.
I say loosely followed here because you don’t want to always text her eleven minutes after she texts you if she usually takes about ten minutes to text you. If you develop a pattern of taking the same amount of time to text her every time you respond back, she’ll pick up on this and know that you are literally just waiting to text her. It’s nearly as bad as texting her right away.