You can connect just about any HDMI device to the HDMI IN port on the 3rd-gen Fire TV Cube, as discussed here in my article all about how the new port works. While Amazon seems to expect cable and satellite TV set-top boxes to be the most common things connected, I expect game consoles will be commonly connected as well. Connecting a game console to the Fire TV Cube makes it very convenient to jump between gaming and streaming, however, be aware that the HDMI IN port adds some additional input latency and there’s an important setting you may need to adjust to reduce that latency.
While I don’t have any professional latency testing equipment, I slapped together a test that I believe does a decent enough job of testing input lag to ensure the latency difference between using the HDMI IN port of the new Fire TV Cube versus connecting directly to your TV. I took a wired USB mouse and wired an LED directly to the right-click micro switch of the mouse. This allowed me to easily see the moment the mouse button is pressed. I then recorded 240 fps slow motion videos of the mouse being used on various devices connected to the HDMI IN port of the Fire TV Cube and connected directly to the TV. By counting how many video frames passed between the LED lighting up and the button click’s action taking place on the screen, both through the HDMI IN port and bypassing the Cube completely, I was able to measure the input lag added by the HDMI IN port.
The accuracy of my testing is limited by the speed of my camera, which is 1 frame in a 240 fps video, or, in other words, about 4.16 milliseconds. That said, the data I collected was very consistent across all the devices I tested, so I’m fairly confident with the results. On average, I found that connecting a device to the HDMI IN port of the Fire TV Cube added about 33.4 milliseconds of input lag versus connecting the device directly to the TV. Given the plus or minus 4 millisecond margin of error, I’m comfortable calling it about 35 milliseconds of additional latency.
The Fire TV Cube 3 has a “Game Mode” setting under Settings > Display & Sounds > Display. The setting description states that “Game Mode reduces lag when you’re using supported gaming apps and connected gaming consoles.” The last bit about affecting gaming consoles indicates the mode applies to the device connected to the HDMI IN port. Sure enough, in my testing, I found that leaving this setting turned off resulted in about twice as much input lag through the Fire TV Cube’s HDMI IN port, or about 70 milliseconds more latency than if the HDMI device was connected directly to the TV. So you’ll definitely want to turn this on, even if your TV doesn’t support auto low latency mode.
It’s very subjective whether 35 milliseconds of additional latency is reason enough to not use the HDMI IN port for something like a game console. I expect that most serious gamers will forgo the convenience of the Cube’s HDMI IN port and connect directly to their TV to avoid the additional 35 milliseconds of input lag. For casual gaming or most other non-gaming HDMI devices, it’s probably not a big deal assuming your TV or other equipment isn’t already adding too much input lag, to begin with.