Собственно, сабж: https://www.phoronix.com/news/glibc-Faster-Hyperbolic-FMA
The GNU C Library's tanh and other hyperbolic functions are now as much as 14~17% faster on modern Intel and AMD CPUs with the FMA instruction support for fused multiply-add operations.
The FMA instruction set has been around for roughly the past decade with both Intel and AMD processors. Only now this week thanks to the work of Intel engineer Sunil K Pandey is there an FMA-optimized tanh function along with atanh and sinh functions.