The article from this morning gave the 5-30% performance reduction figure. However, here is an updated statement:

' “Intel has begun providing software and firmware updates to mitigate these exploits,” Intel said in a statement. “Contrary to some reports, any performance impacts are workload-dependent, and, for the average computer user, should not be significant and will be mitigated over time.” '

So, you're right. It's easy for the media to misinterpret numbers and run away with their conclusions. Based on what you're saying about the exact mechanism behind the bug, it makes sense that potential slowdowns will be highly dependent on workload. For scientific calculations running on e.g. BOINC, I wonder if flushing the CPU cache between 'user' and 'system' permissioned processes would even really matter. This is def going beyond my area of expertise. I'm going to track with the developing news though, and hopefully we eventually get a clearer picture of what the impact actually is.