Slow transfer

My son is 3 weeks 5days and takes an hour and a half to transfer 1.5 oz and then I'm still able to pump an oz. We have been supplementing since he was 3 days old due to him losing over 11% of his birth weight. When I don't put him to breast he eats 3oz of formula/breastmilk every 3 hrs. How can I get him to transfer quicker and be able to wean from formula. We've seen an LC she was really no help other than reassuring that he has a good latch.

Breastfeed, supplement, pump, repeat 😣