A make several 60 x 72 quilts a year for kids going through confirmation at church. At first it took me 4-6 hours just to do the quilting on the sewing machine. Now, about 2 hours. I did one last week and I sewed about 1/2 hour took a break and that is about the way I quilt best. Breaks help ease any tension I may get in my shoulders. All added together I was so excited to complete this one in 2 hours. I do a diagonal. I have done so many that I do not premark the lines, now. I use a disappearing pen and mark only the lines in the borders to connect to the first square block so I get my beginning angle correct. This was my fastest ever. I do not get bird's nest. My back looks as good as the front. I use a Janome 6600P. My speed just gradually got faster as I learn how to hold the quilt as I stitch. Music is relaxing also.
Different speeds for everyone. I personally don't quilt to get it done quickly. I enjoy the process, buying the fabric, planning, caressing the fabric etc.....