I tried to implement what you said on my M1 2020 MBP. Heres's the one liner (albeit using 0% of CPU as the cutoff, and only throttling Safari processes):

  ps axo pid,%cpu,command -r | awk '{ if ($2 >= 0 && $3~/WebContent/ )   { print $1 } }' | xargs -L 1 taskpolicy -b -p

You could set this up to run at whatever interval, and it should work. Using browserbenchmark.org, before running this one liner I get a score of 274. Afterwards, I get a score of 33.6.