Recently learned about Ghostscript and man, it does PDF manipulation really fast. If you find yourself merging PDFs a lot, here you are:

    gs -q -sDEVICE=pdfwrite -o merged.pdf tobemerged1.pdf tobemerged2.pdf tobemerged3.pdf

Ghostscript is similar to ImageMagick in that it does so much that learning to do one specific thing is hard. But that line merged ~300MB of PDFs together in 20s on my M1. Doing that in Preview causes a beach ball.

I installed it with Homebrew, but the project home is yonder: https://www.ghostscript.com/