They generally tend to slow down after about 6-8 months, if that's any help. And it's normal that they do, although people will start to say things about growth slowing down.

And YoungBloke was in the highchair before he was able to sit up unaided. I suspect the highchairs have their notices so that people don't leave babies in them for any length of time or they slide out of the sides or something. You could try feeding her sitting on your lap although sometimes you'll feel like you need a couple extra pairs of hands if you're trying to do it all yourself!