Check your foot pressure. The fabric can be pulled between stitches if there is not enough tension on the fabric. I often sew wide open with my Juki, and never notice a difference. However I prefer to sew with a smaller stitch than most.
However, just about everything is going to be exaggerated when you go high speed - it's a good way to make sure everything is adjusted correctly. I've seen many machines that appeared to be adjusted correctly - when sewing slowly on heavier fabric. Switch to fast on lighter weight fabric, and it was obviously not.
BTW - that is how a gather foot works, it is formed so there is no pressure on the fabric behind the needle, so when the fabric is fed, it allows the thread to pull tight and pucker up the fabric. You can adjust the tension and stitch length to affect the amount of pucker, but it's the lack of tension on the fabric that allows the pucker to form in the first place.
Last edited by Macybaby; 09-29-2016 at 03:40 AM.