Your BenQ projector supports one frame latency when its Fast mode is activated only under the following timing with the Keystone set to "0" and the Aspect Ratio set to "Auto".
The latency of one frame will vary depending on the timing of the input source. See details as below:
(1) 16ms latency at 4K@60hz
(2) 16ms latency at 1080p@60hz
(3) 8ms latency at 1080p@120hz
(4) 4ms latency at 1080p@240hz
*Image processing like Scaling, Keystone, or Aspect Ratio setting will have up to 2 frames impact.