The native Garmin software handles creation of record messages, the interval at which they are created, and creation of the timestamp in the record message. As previously stated the interval can be controlled by the end user, but the default is 1 second record message intervals in most cases.
The CIQ app simply adds data to the next record message to be logged. This means that your data may in the worst case be logged and timestamped a second later than you made the call to log it.
With ANT there is at least a 1 channel period delay between when a measurement is made and when it is received by the watch. Depending on how the profile sets up the transmission pattern it may be more. Note that ANT is lossy. Every channel period a message is sent, but it may fail due to poor RF conditions. This leads to a data stream that is typically 1 channel period latent and has gaps.
Depending on the sensor algorithm there may be additional latency between when the measured value is determined and the time for which it was applicable.
End to end between the sensor and FIT record message you have
sensor algorithm latency + 1 ANT channel period minimum + up to one FIT record message period
