I've run relatively loaded unbound in the past - I would suggest to use minimum num-threads with which a single thread uses less than 50% CPU at peaks. And set number of slab to the same value (if it is power of 2) or lower. High number of slabs increases memory usage (may be it can grow over long time because of fragmentation, but I've not noticed this). Unbound Howto Optimize [1] suggests to set num-threads equal to number of cores but IMHO it makes sense only if: server runs no other software except Unbound (even in this case some cores will be utilized by the kernel so better give to Unbound less than total core count) and it has more than enough RAM so possible memory fragmentation is not a concern.

[1] https://www.nlnetlabs.nl/documentation/unbound/howto-optimis...

unbound-1.14.0 (newest).

But I should have qualified -- I run a caching resolver for use by half a dozen users (so about 20 devices). Load is negligible, but it works perfectly! YMMV.

As an added benefit, my unbound instance is also faster than 1.1.1.1, 8.8.8.8, or my ISP's resolver farm.