Ever wondered why you can't hit that pesky light tank that is trying to circle you to death? Well, I have...
I couldn't shake the thought of how silly this is and in real life my shell would reach the target nearly instantly. There was even a time I have put my tinfoil hat on and thought the lag is intentional and is there to prevent RB + RT (autoaim) kills. It turns out there is a much better and simpler explanation. So, by the power of Occam's razor, let's cut to the chase.
Let's assume the following parameters in this theoretical scenario:
Hardware
Server lag: 100 ms (bad case scenario)
Input latency: 70ms (Xbox is known for having a 3-6 frame delay on 60fps games; yes, I know WOT runs on 30fps due to 360 reasons)
In game
Light tank speed: 50 km/h
Light tank length: 6 metres
Distance to target: 50 metres
Shell velocity: 1000 m/sec
Now, let's calculate how this would play out.
Shell travel time @ 50 metres with 1000 m/sec is 50ms. Plus server lag and input latency, we get 220ms total delay (nearly a quarter second) between trigger pull and shell reaching its target. Tank travelling at 50 km/h moves 3.06 metres every 220 miliseconds. Given that most light tanks are shorter than 6 metres and Autolock aims at the centre of the mass, by the time shell reaches its mark, the light tank is no longer there and shell flies behind it. I didn't even factor in the gun's aiming time and/or dispersion, as it doesn't matter. Even if the gun was perfectly accurate, it would still miss the tank aiming at centre of mass. (Actually, you have a better chance of hitting your tank on a miss in this case, since the shell may deviate left or right from dead centre, giving you more deflection) 
This is why it is necessary to lead targets that are travelling perpendicularly in relation to your tank, even at distances as low as 50 metres. Of course, the amount of lead depends on individual conditions and will differ greatly between you hardware, server connection, tanks used, etc..., but the concensus is you cannot simply Auto Lock on those targets. You have to aim manually.
Not sure if this helps anyone, but it sure put my mind at ease. Back to storage with you, tinfoil hat!
Good luck & good hunting, tankers!
PS. If you think this is unrealistic and 220 milliseconds is too long, blink twice. That's right, a literal blink of an eye is between 100-150 ms.