Older versions of the Linksys AC1900 EA7500v2 firmware for this router performed poorly with Circle, and our testing has determined that updating the firmware can mitigate this. See this article from Linksys on performing firmware updates. If a firmware update is not possible, speeds can be greatly increased by connecting Circle to Wi-Fi rather than Ethernet.
Other possible workarounds
Enabling compatibility mode for Circle Home Plus may resolve some of the above issues as well. You can read more about compatibility mode here.
Compatibility Mode is not available on the Circle 1st generation app or Circle with Disney devices.