Speedweaving is more than just moving guests, and I see many new GSA's struggle with it, as well as WHEN they should be doing it.
#1 Priority) Move guests to shorter-wait lines, if you have a guest waiting in a line that has a longer wait, but have another cashier who is finishing up, ask them if they'd like a shorter wait in the other line, and move them there. If guest is unhappy with wait, call a backup and offer them another lane to get checked out right away.
Secondary Priorities) Loop the fronts of the lanes, and then through the back of the lanes. If you see any speed bumps (hangers, stuff under carts, bulky items), help out with those. Collect baskets/strays/clutter and be interactive with your guests/cashiers. You are also reducing speed-bumps by just "being there" in-and-out with your cashiers; if they encounter a problem you are right there to fix it to keep things moving quickly.
During busy periods you should be speedweaving, but its less vital when its slow, and those are the times to get other stuff done.