Diamond blade plus angle grinder on a treet stump killed someone
recently (kickback into neck). Suicidal.

You can use a cheap jigsaw with wood ripping blade if you cut them
over a table. Very quick unless stuff is 2in thick.
You can use a reciprocating saw - like a normal handsaw but two blades
driven like hedge trimmer blades, but they cost somewhat more (better
for chopping thro old studwork).

I would not suggest even a handheld circular because of the risk - you
are working "sloppy-fast" which could result in kickback or accidents.
The jigsaw would probably be best for thin cladding (shiplap etc), a
lot quicker than you realise.