If the CPU has working AVX (i.e., Intel Sandy Bridge or later. No AMD at all.), run any of the LLR projects. Make sure you have hyperthreading turned off, or tell BOINC to use only 50% of the cores.
If your CPUs don't have AVX, or they're AMD CPUs, run the TRP Sieve or the GCW Sieve. With these projects, it's okay to use hyperthreading.
Another project you can use old CPUs on are the small GFN projects. GFN15, GFN16, and both GFN17s. AVX doesn't help with those, and GPUs aren't as efficient on the small GFN tasks, so it's a good place to use your non-AVX CPUs.
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