Is that really the case?
If I understood you correctly, your previous ISP didn't require PPPoE, but worked over plain ethernet instead. Especially at high throughputs, PPPoE is a significant CPU hog, much more so than plain ethernet. It doesn't surprise me that your plain ethernet throughput is considerably faster than PPPoE, but by comparing the maximum routing speeds against each other, you're comparing apples with oranges.
IPQ806x (and I'm already basing this on the faster ipq8065 SOC, instead of your slightly slower ipq8064) is generally said to be good for around 400-450 MBit/s routing throughput, not more. Your ea8500 however adds another hardware obstacle. Contrary to all other known ipq806x devices, Linksys opted to only connect a single CPU port to the internal switch (instead of two, one for WAN, one for LAN), which reduces the theoretical throughput even further, as all traffic has to pass it twice (WAN <--> CPU <--> LAN, so even ignoring the SOC's actual performance, you're down to 500 at most, due to this unfortunate hardware design choice). So your figures are already better than expected for ipq8064 in general and the ea8500 in particular.
For more details, please see:
Kernel 4.19 (as used in current master snapshots, be aware that snapshots don't have the webinterface/ luci preinstalled) improves the maximum routing throughput significantly, but probably not enough for your needs either.