Depends on how long the slots are to be. *If not too long, a table saw would
be worlds faster, you could stack the pieces and cut slots in each at one
time...position stack, cut, position stack, cut. *Et cetera.

When I built my last torsion box affair (served as a six+ foot shelf/
table top) the internal cross members were relatively thin (quarter-
inch or so) and the slots were half their width (less than half the
thickness of the finished torsion box).

I think a ten-inch TS will cut in excess of 3" at ninety degrees. Less
with a DADO blade.

But, as someone pointed out already, the OP provided insufficient
detail from which to determine a best practices approach the project
at hand.