It’s mostly power throttling.
The R9 Nano has a decent amount of thermal headroom, but will power throttle fairly quickly. Simply increasing the power limit in Wattman locks the card to the 1GHz base clock. Once you’ve removed that power limit then you’ll find some games that will cause it to thermal throttle (i.e. anything that demands 100% GPU resources regardless of load like World of Tanks or ARMA III), but a pretty minimal increase in fan speed will keep that from happening.
The R9 Nano sets a lot of the foundation for how Vega is overclocked. Power limit up, voltage down. It will happily push up toward 1100MHz with a bit of extra fan speed, a minor undervolt, and a maxed out power limit. (Though in synthetic benchmarks you’ll run right into the power wall and see regressions in performance usually. 1050 - 1075MHz is usually where the the balance point is.)
It’s a good card still. Honestly it’s still usable for most games up to 1440p so long as you aren’t pushing massive AA at the same time. The rough modern equivalent in performance is the RX 590, though amusingly enough the R9 Nano uses less power.