Same story here, just slightly worse (the ea8500 is ipq8064 based, while the nbg6817 would be ipq8065; additionally the ea8500 only uses a single CPU port to the switch, which limits the theoretical full-duplex bandwidth even further). In short, that's about the expected throughput range the ea8500 can master, you're pushing it beyond its abilities with 1 GBit/s WAN speed (if you can build OpenWrt from source kernel v4.19 might improve the situation a little, but not all the way).
Routing at 1 GBit/s linespeed still dabbles into enterprise territory, you need matching hardware for that, which basically boils down to x86_64 or mvebu.