I leave the baby in the high chair. He has to learn to be patient and this is the only way. I give him food slowly too, a bit at a time so he doesn't choke it all down at once. If he's getting fussy and ready to leave and he's eaten a sufficient amount I will usually throw a cracker or something he can handle his way to keep him occupied a little bit longer.
In my house, you leave the high chair when I say not when they say!