Can this NAS be integrated with existing iSCSI targets, and are there any specific network requirements for optimal performance?

Yes, this NAS supports integration with existing iSCSI targets as it features the iSCSI Target Protocol.

What kind of Ryzen CPU is used in the Synology DiskStation DS1621+ NAS Server, and how does it impact performance?

The Synology DiskStation DS1621+ NAS Server uses a Ryzen V1500B Quad-Core 2.2GHz CPU.

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Synology DiskStation DS1621+ NAS Server for Business with Ryzen CPU, 32GB Memory, 24TB HDD, Synology DSM Operating System, iSCSI Target Ready

Ryzen V1500B Quad-Core 2.2GHz 4MB CPU; 32GB DDR4 PC4-21300 2666MHz SO-DIMMs; File Storage: 24TB (6 x 4TB) 6Gb/s SATA 3.5 Inch HDDs for High Capacity Storage; 4 x RJ-45 1GbE LAN Port (with Link Aggregation / Failover support); 3 x USB 3.2 Port; 2 x eSATA Port
Features: iSCSI Target Protocol, Cache Acceleration (2 available M.2 SSD Slots), Virtual Machine Manager, Synology High Availability, Synology MailPlus, Central Management System, Security Advisor, AES 256-bit Encryption, 2 Factor Authentication, Cloud Station Suite, 4K Multimedia Server role, Active Backup for VMware and Windows, Synology Drive, Active Backup for Office 365 and G Suite, Surveillance Station role, Collaboration Suite, and many more
Synology NAS chassis comes in a sealed box.
Hard drives and memory upgrades included separately NOT installed, installation required.