you hit the nail (rather pin) on the head when you said "slow down" Fast and speed is the grimlin in quiltmaking.
Also, have you noticed how the feed dogs grab the fabric at the end of the seam and veer to left making the seam at a critical point smaller. Or is it to the right....
I took a class from a very fine quilter who does small quilts. The thing I noticed watching her was how slow she goes and used something to help guide the seam. I tried it and it works. There are certainly some long seams that you can go faster but mostly just slow the dumb machine down and sew straighter.