https://ark.intel.com/content/www/us/en/ark/products/202777/...
There's no mixed instructions as Anandtech speculated. The Sunny Cove core was cut down to match what the Tremont cores support. So no AVX at all, no weird extension mismatch, no OS headaches beyond the expected big.LITTLE headaches.
Anecdotically, I have a "netbook" that has a Goldmont+ Celeron N4000 that works very respectable for everyday business (web browsing, office suites, watching videos, etc) but crawls when trying to run scientific code. 100x slowdowns at worst, 10x slowdowns on the parts that take "nice and easy" (wrt a 7200u notebook that I usually carry around).
You mean not much. The question was apart from scientific computing how much does it hurt. Per your own comment it's only worth maybe 20%, and that everyday needs work fine.
It'll show up occasionally in some things that would be relevant to a device with this SoC, like noise cancellation, but very little else. And even then you can do noise cancellation even better on a GPU (RTX Voice says hi), and this does still have a GPU and a decent one at that (~500 gflops), so it's not even that simple.