If I restrict the Raspberry Pi 4b to a single hdmi channel, will that affect/improve animation performance?
If your current setup uses an extended display, limiting the RPi’s display configuration to a single display would definitively improve performance.
Setting the RPi to mirror the same output onto both HDMI should also achieve the same result because the resolution to process would require half the bandwidth than “extended”, and is re-used for the second display.
Nevertheless, for best results across all computers you should use a video splitter that takes one HDMI signal and outputs two or more identical signals to its HDMI outputs. This approach may add a small consistent lag to the response time, but lifts work away from the source computer, reducing jitter and eliminating potential v-sync issues present when using an extended display.