For reference, my server CPU is an AMD Ryzen 7 PRO 5750G (65W TDP, 3.8-4.6GHz, 8-core, 16-threads) and CPU usage is around 8% when writing at 700 MB/s on my rai

For reference, my server CPU is an AMD Ryzen 7 PRO 5750G (65W TDP, 3.8-4.6GHz, 8-core, 16-threads) and CPU usage is around 8% when writing at 700 MB/s on my raidz2. In other words, only a little more than 1 of the 16 hardware threads is utilized to handle this workload. Your older Xeon D-1521 should be able to handle that, but it's true that being an older processor, it is pushing it closer to its limits. Handling concurrently 10 Gb/s of network traffic would likely push it to more than 50% utilization. That's one reason I like to not cheap out too much on the CPU, especially for a 10GbE ZFS NAS.