zatyka
Of course, repeated AJAX requests is out of question. But syncing the game clock only once upon loading is not really helping to validate a timed high-score. You need two requests, for starting time, and ending time... so twice the latency. Depending on the game need, it might not be an option.
I guess combining both external and internal validation methods could work too. Compare external time with in-game time, and if results are too far apart (latency excluded), reject the high-score.