“This can make cache timing attacks a lot easier and we strongly suspect that this will make several Spectre-class bugs exploitable.”The performance impact depends on the use case. Some applications benefit hugely from Hyper-Threading, while other workloads actually see a performance hit. A lot depends on software optimizations, and not everything is suitable for parallelization.
So OpenBSD has decided to disable Intel CPU hyper-threading, with “a new hw.smt sysctl,” in order to avoid data potentially leaking from applications to other software via Spectre-like processor flaws.
The measure isn't limited to Intel CPUs. Kettenis writes OpenBSD plans to extend the feature to CPUs from other vendors and other hardware architectures.