Both single and multi core performance are important in different ways.
This is an oversimplification, but single core performance roughly determines how well any individual camera will work. Faster single core performance means you can run a camera at higher resolutions and frame rates with less chance of frames being dropped during playback. In other words, faster single core helps
Blue Iris maintain smooth playback. Multi core performance basically just determines how far you can scale up by adding more cameras. As such, once you have "enough" multi core performance, you really don't benefit from having more.
When Blue Iris started supporting "dual streaming" (sub streams and main streams working together), this feature drastically cut down on the need for higher multi-thread performance because each camera you add with sub streams properly configured will only increase the total CPU usage by a small amount. Because of this, multi-thread performance doesn't matter very much anymore unless you can't use sub streams for whatever reason.
But single thread rating is still just as important as it ever was. A 4K @ 30 FPS video stream can be very resource intensive to play through Blue Iris, especially if you are streaming it remotely through the web server because that means Blue Iris needs to encode a 4K video stream at 30 FPS in realtime and that depends a lot on single thread performance.
If you want an upgrade, the most significant aspect to upgrade is single core performance. Better multi core performance will usually come along with it automatically.
For example, if you have i7-8700, you already have single core performance that is comparable to a lot of recent high-core-count server CPUs, so "upgrading" to one of those server CPUs would probably not be much of a benefit but it would increase your power consumption and noise levels. Blue Iris doesn't even support multi-socket servers properly (it is not NUMA-aware) so multiple sockets for the sake of Blue Iris is largely pointless.
| Processor | Multithread Rating | Single Thread Rating |
|---|
| Intel Core i7-8700 | 12,787 | 2,633 |
| Intel Xeon E5-2697A v4 | 21,676 | 2,102 |
| AMD EPYC 9845 | 152,985 | 3,144 |
| AMD Ryzen 5 9600 | 30,002 | 4,578 |
Don't get distracted by huge multi-thread ratings. For Blue iris, the Xeon v4 is arguably the worst of the options I presented above (doubly so if you run two of them!). Ryzen 5 9600 is by far the best choice of the 4 processors in the table above, because it has much better single thread performance and better energy efficiency for the job it would be doing. I'm not saying Ryzen 5 9600 is an ideal Blue Iris CPU; it was just a cheap example of a CPU with high single thread rating. If I was building a "new" PC for Blue Iris I would probably pick an 8 or 12 core Ryzen 9000 series just because the added cost of the better CPU is not much compared to the cost of a new case, power supply, motherboard, RAM, etc.
