Re: OK, I'll bite
it has been stated that you won't experience slowdowns unless you are doing a lot of disk access or network access
That may have been stated (by whom?). That doesn't mean it's correct.
The Meltdown remediations cause a performance hit for all kernel-to-user context switches. I/O is a major cause of such switches, but it certainly isn't the only one.
Software-based Spectre remediations, once they start appearing in software, will cause a performance hit every time they're encountered. As hardware assist for them is introduced in new CPUs (such as the new conditional branch instructions ARM describes in their Spectre whitepaper) the cost will drop, but for older CPUs the techniques that have been identified so far, such as memory fences and retpolines, are significantly expensive.
 Aside from the initial stopgaps put into browsers, which were simply to defeat the two cache-timing mechanisms identified in the Spectre paper.