Advice needs: kitten adoption and resident cat not happy

I have an 8 year old resident cat who is a rescue and now adopted a 16 week old kitten. We have had the kitten 8 weeks now, have completed scent swopping, seperate rooms and I have a mesh gate to seperate them. The resident cat is still very stressed by this new additon, she has urinary frequency and is hissing at the kitten everytime she sees her. I can’t get both cats to eat with each other and my resident cat never asks for food so would not work. How long does it take for both cats to at least tolerate each other? Is there anything else I can do? Thanks In advance