First of all - PC spec: Threadripper 2990wx, 128 Gb memory, GTX titan X. All devices not overclocked. Only stock.
Problem - mantra render work slower when all 32cores (64 threads) is active. But! When active only 16 cores (32 thread) mantra render work faster. 32 cores - 2.54 min, 16 cores 2.11 min. At same time Arnold render, in Maya 2018 update 4, work more predictably. More cores - more speed. 32 cores 1.26 min, 16 cores 2.17 min. Screenshot and test scenes in attach.
UPD. Interesting situation with SMT (simultaneous multithreading) technology. When SMT off with 32 active cores, mantra work much faster - 1.53 min vs 2.54 (SMT on).
When active 16 cores and SMT off - 2.30 min. vs 2.11 min. (SMT on).
Bottom line.
Mantra - 16 core, SMT on - 2.11 min.
Mantra - 16 core, SMT off - 2.30 min.
Mantra - 32 core, SMT on - 2.54 min.
Mantra - 32 core, SMT off - 1.53 min.
And after some tweaks in bios (not direct CPU overclocking)
Mantra - 32 core, SMT on - 2.19 min.
Mantra - 32 core, SMT off - 1.35 min.