It's likely a hardware issue, i.e. there's only so much CPU capacity.
On 2900 series ISRs, throughput varies very, very much based on different factors. The two main factors are the size of the packets and what additional functions are performed on those packets as they transit the router.
The white paper provided by Balaji show the 2921 has been shown it can achieve a throughput of 3.5 Gbps, but that's when processing all 1500 sized packets, and doing nothing but basic forwarding. I.e. performance you'll likely never see in the "real world".
Later tables in the same document show performance applying different "services", for an "Internet mix" of packet sizes and not exceeding an CPU average of 75%. For these, you see a huge drop of performance, down in the low 100+ Mbps range.
Because Cisco doesn't want you to run out of performance, they advise (also shown in a Balaji post) to only use a 2921 on 50 Mbps WAN circuits (such circuits are duplex, so they are advising to plan on not obtaining more than 100 Mbps, in aggregate). However, can you obtain more than 100 Mbps? Possibly yes (as shown by your speedtest results of 280 Mbps), but how much more is a very much it depends question.
For 500 Mbps, you'll want/need a router with more performance/capacity.
