Will ECC RAM bottleneck my TrueNAS transfer speeds?

No. ECC RAM is not a speed bottleneck, especially for a 4x HDD TrueNAS build. Your hard drives are the bottleneck—four spinning disks max out at 500–800 MB/s sequential, well below what any modern DDR4 or DDR5 RAM can handle. ECC adds a tiny latency penalty (1–2% in synthetic benchmarks), but it’s invisible in real-world file transfers. Your transfer speeds depend on your HDDs, network, and SATA controller, not the RAM type.