My impression on systems from the days before linux/Intel began having real SMP is that if you are willing to go off the rails completely on supporting 1-2 core

My impression on systems from the days before linux/Intel began having real SMP is that if you are willing to go off the rails completely on supporting 1-2 core machines and making every CPU available to every task, then blocking side channel attacks no longer requires all the work.. If mitigation is starting to amount to ~20%, that's roughly a CPU and soon to be 2-3, that can be told to never process directly for Userland and never tell Userland when exactly output queue contents of sensitive tasks become available. All that work essentially being free compared to running with Intel's fixes and full CPU flexibility.