It’s too early to tell what the broad performance impact of the latest Spectre mitigations will be, but so far it’s clear as mud. Thomas Claburn wrote this for The Register:

The performance impact when Hyper-Threading is disabled is dependent on many factors. Measured impact ranges from a +30 per cent gain, to -50 per cent loss and beyond. Most HT testing, however, showed losses in the 0-30 per cent range.

We’re dealing with this at work at the moment. Fun times.