The figures quoted here are estimates based on theory then confirmed with experimental testing of serial communication only.
The image below details the internal processes within an OxTS product running rtbns firmware with a generation seven IMU (late model RT3000, etc.).
In order to confirm the numbers above a test comparing the time difference between the PPS signal and the serial output was devised. Working through the diagram the latency between these two signals ought to be the sum of a portion of the digital processing time (5 ms), the navigation time (1 ms), the serial output preparation time (0 ms) and the serial transit time (up to 5.6 ms).
The oscilloscope read out below shows that the actual time between the PPS signal (blue trace) and the serial message (green trace) is 3.22 ms.
In this case the serial transit time is likely to be close to 0 ms, subtracting the 1 ms for the navigation time leaves 2.22 ms for the digital processing (in line with expectations) from the point of PPS input.