Performance drops after a couple of minutes of compiling.
Thermal throttling. Check the sustained clock with a monitor running, then decide between undervolting, a lower power limit that trades peak for consistency, or a cooling pad. The consistent lower clock usually finishes builds faster than the sawtooth.
Is ECC worth it for a home NAS?
For a filesystem that checksums data, ECC protects the one place checksums cannot: memory between verification and write. It is not mandatory, and the honest framing is insurance whose premium is a slightly more expensive board.
Which drive survives a write-heavy workload?
Look at TBW rather than sequential speed. A consumer drive rated at 300 TBW dies years earlier than one rated at 1200 under the same load, and the fast numbers on the box describe bursts to cache, not sustained writes.
How do I know when it stops being worth it?
When you stop noticing it. If it has not surfaced a problem in six months, it is either solved or it was never the bottleneck you thought.