Yes, the actuall 1541 interleave is 10....but jiffydos and super snapshot lower that figure. Note that the 1571 in double sided mode interleaves at 6. Any time a different interleave is expected by the equipment than the one on the disk, there is a slowdown. The Maverick V5 file copier lets you designate the skew or interleave for the destination disk independant of the source files.
One thing I find annoying is 128 mode programs SDA'd in 64 mode for this reason. I discovered that Maverick will allow me to change the skew, and I really did notice a BIG improvement. My Point: Make sure to use THE right skew for you!