PPPoE capabilities on the Time Capsule are extremely limited.
Using an Ethernet connection and DHCP service from my modem.....not PPPoE......the Time Capsule checks out at about 325 Mbps when it is set up as a DHCP and NAT router. I have a 500 Mbps connection from my Internet Service Provider.
This 325 Mbps reading is about the same as I have seen in most test reports on the Time Capsule that appeared 6-7 years ago. 325-350 Mbps was not bad in 2013-2014. Very few routers were capable of providing 500 Mbps speeds at that time and there were very few Internet Service Providers that could deliver that level of speed at the time.
But......Apple never changed or improved the hardware in the Time Capsule from the time that it was introduced in 2013 to the time that it was discontinued in 2018. Thinking further..... if the Time Capsule was introduced in 2013, it was designed in the 2011-2012 timeframe using technology that was available at that time.
When DHCP and NAT are turned off, and the Time Capsule is set up in Bridge Mode, you could expect 700-800+ Mbps speeds over Ethernet and Wi-Fi, assuming your ISP can provide those numbers.
Yes, you will need to use the Time Capsule with another router if you want 500+ Mbps speeds from the Time Capsule.
But, if your other router is delivering 500+ Mbps speeds, I would use the other router for your network and connect the Time Capsule and use it only for Time Machine backups. In other words, turn off the Wi-Fi on the Time Capsule since you will be using that service from your other router.
Tell me, please, what could be the problem?
Frankly, the Time Capsule is a decade behind at this point, technology wise.