Quick Testnet How-To For Windows
Replace with your address and <# of threads> to a maximum of the amount of L3 cache divided by 2; e.g. - a Ryzen 5 1600 has 16MB L3 cache so max number of threads is 8 (but to preserve desktop interactivity set to 1 or 2 less threads than maximum).
I'm not sure this is good advice for all cases. I have a 44 core machine with 55 MB of L3 cache. Mining with 27 threads (55 / 2) I get about 62% of the hashrate I get by mining with 44 threads. (I have turned hyperthreading off, so have not tried with 88 threads, but I doubt that would see better performance, and it could well be worse.)