It appears that all modern Intel processors contain a hardware-level security flaw [theregister.co.uk]. Details are being suppressed while patches are developed, but it appears that a user process can put a reference to a privileges address in speculative execution, and thereby bypass privilege restrictions.
Since simply marking certain areas of the virtual memory space as privilege is insecure, operating systems will have to completely isolate their kernels, i.e., remove them from the virtual memory space of user processes. This will make context switching more expensive. Presumably, every OS call will now require swapping out the virtual memory map and flushing the page tables cache in the processor. Twice: one to go to the kernel, and once to return to the user process. This will be a significant performance hit: depending on the application, up to 30%.
This is apparently an Intel-specific bug; processors by other manufacturers are not affected. However, they may still suffer the performance hit: The changes to the OS are substantial, and it seems unlikely that OS manufacturers will, in the long-term, maintain two completely different kernel-access strategies for different processors.