If speeds are noticeably slowed using this router with Circle, 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 Wifi, instead of Ethernet.
Other possible workarounds
Enabling compatibility mode for Circle may resolve some of the above issues. You can read more about compatibility mode here.
Compatibility Mode is not available on the Circle 1st generation app or Circle with Disney devices.