Hi, i am currently work on my thesis on 3D-Renderfarms and do lot of researches in any direction. So this came up during my researches of threads and their speedup imapct.
When it comes to threads, blender will verify Ahmdahls Law pretty accurate. The first 35 % of threads will have something like 80 - 85% boost impact. The last 30% of the threads will boost your render like 8 - 10 %.
So if you have a CPU which natively supports 32 Threads, the last 15 threads are adding not so much to your render speedup.
That means, for all who work and render on the same machine, a decrease of the threads by 1 - 2 will not hurt your render and still let you work fluently through your day (depending on your logical processors).