You can set MAKEFLAGS in /etc/makepkg.conf to something like "-j8" (where "8" should be something like the number of cores you have or maybe number of cores minus one or two if you want to leave some CPU capacity available.
However, the build instructions for a specific package can override these defaults. You'd have to look at the resolve-davinci package files to see if it does that for some reason that might be important.
Better yet, use ~/.makepkg.conf to avoid setting it system wide and use MAKEFLAGS="-j$(nproc)" to have it automatically detect how many processes to use.
Pretty sure what you were actually compiling was qt5-webkit, which davinci-resolve depends on, and yeah its a bitch of a compile without multi-threading.
There's a file, I can't remember what it is but I have it written on a sticky note on my desk at home, where I edited the makeflag or something to use 10 threads instead of 2 (I have a r5 3600) so maybe you just aren't using all your threads.