With additional variants of Spectre circulating, Intel has taken an initiative to circumvent exploitation and (with any luck) stop future Spectre variant exploits. On July 2nd, 2018, Intel released a new series of "hardening" microcode to support processors dating all the way back to Westmere (first generation Core). If you would like details on microcode update releases, you can view the full Intel report here: https://www.intel.com/content/dam/www/public/us/en/documents/sa00115-mic...

Given these updates, it is yet to be seen what kind of performance impact you can expect. Further testing and detailed benchmarking will be required to determine what kind of I/O decrease should be expected.