Today, I did a little test. Started with a RWGPS road route I created,
STOPPED at the start of each "turn" and the results can be seen below. The turn cue lagged on average 29 ft, with one being off 92 ft.
Okay, not so bad... but what happens as you approach the turn and are NOT stopping... on average is was lagging by 74 ft, with a max discrepancy of 93 ft.
To me that's a large error, at the point you need to start turning its way off.