The Intel Processor vulnerabilities seem to be a giant fiasco. Apparently the fix slows the system by 30%. Which would also affect renders, obviously.
Fortunately the high performance hits appear to mostly be on server-oriented workloads. We haven't tested Terragen extensively on patched systems yet, but judging by performance impact (or lack thereof) on other renderers we don't expect to see a big issue: https://www.techspot.com/article/1554-meltdown-flaw-cpu-performance-windows/
We have to file for refunds! :P
One note I will add to this based on a bit of discussion I have seen in VFX circles: if a scene involves a lot of big textures or anything of large size loaded off of disk or off the network, it could impact rendering performance. In most cases Terragen loads everything into memory before rendering, so if there were to be a hit I would guess it'd be in scene load time. But I think this is more likely to affect those renderers that try to be more clever with mipmapping, disk caching, etc. Terragen's simplicity is a strength in this case. :)
I won't update anyway, as I work offline. ;D