Have you considered adding a suspension system for the quilt? Suspension systems keep most of the quilt off the sewing surface so it doesn't add drag. It also helps your shoulders and arms, because you don't have to move the entire quilt around as you work. Here is a link to a commercially available system, but there are also DIY instructions online. Some quilters, such as Leah Day, suspend their quilts from the ceiling.
http://www.jennoop.com/suspenders.html
https://www.youtube.com/watch?v=gJGph8am6BE