That's interesting. 87u can do up to 60Mbps with average 50-54Mbps on optimized channels if overclocked. You were running your vpn on non optimized channel but 70mbps is impressive. That linksys has 1400Mhz so it's higher then 87u stock 1000mhz. CPU is what does vpn so the better the cpu the faster speed will be.
Unfortunately most if not all routers are ARM cpu which suck balls. They etiher need to do quad core or somehow increase througput. That atom cpu would be killing it. It has up to 8 cores and runs up to 2.4Ghz per core while using 15-20W of power. Amazing.
For $250+ u might as well build atom pc 25w power usage and put pfsense on it as router, either buy some wifi cards as AP or use cheap router in AP mode. Pfsense don't support AC mode yet but then again who cares. Using cheap AP like 66u would be ideal. I got my 87u for $44 so i can't complain about pricing and what i get for it.
Multiple CPU cores wouldn't help, since OpenVPN is single-threaded. What we need is hardware crypto support, such as AES-NI support which is available on the ARM Cortex A53 CPU.