Connecting Two GS116Ev2 Switches Best Practices

> [...] One of the switches which is located in the office upstairs and
> connected to the R7000 router [...]

   Presumably to one of the LAN Ethernet ports?

> [...] and the other is located on the first floor. [...]

   And where's the router?  As a general matter, I'd try to run a
separate cable from the router to each of the switches, but I wouldn't
expect any serious trouble if the arrangement were router -- switch --
switch (instead of switch -- router -- switch).

> [...] I checked the cables on each of the devices, [...]

   Did you look at the port-status LED indicators (which should indicate
the link speeds)?

> [...] I was getting a message on my AppleTV 4K stating the I was
> unable to playback content smoothly from my PLEX server because my
> network was not fast enough. [...]

   And where are those devices connected?  I'd try reducing the hardware
between these devices, to see if you can assign blame anywhere.  But I'd
be a little amazed if network switches were the source of any such
problem.  I'd worry more about your (unidentified) "my PLEX server", for
example.