A handheld circular saw with the depth set to just cut through the ply
does the fastest neatest job. You can make a half dozen or more 8' cuts
in gthe time you'd make one with a jig saw and it would be a lot smoother.
Evan Gatehouse wrote:

On the first stitch and glue dinghy I built, I did it slow and
carefully with a jigsaw, and a block plane to finish.

The second I used just a jigsaw.

The third I used a jigsaw and tried to go as fast as possible. Errors
greater than 1/4" were trimmed with the jigsaw. But otherwise it all
got hidden in the tape seam anyway.