I read the network part a couple days ago.

If two people are playing on a server, playerA has 150ms ping and playerB has 25ms ping, does it take 150ms for playerA to notify the server of any location/health changes, then another 25ms for the server to notify playerB of those changes? Meaning any status changes from playerA take ~175ms to take effect on playerB's screen?

My point is that if someone with a 100ms+ ping plays against a low ping player, there is a disadvantage to both players?

No, the real changes take place on the server and this is what counts when evaluating game physics. So
When player A presses a key to dodge it takes 75ms before he actually moves (even though on his comp it looks like he moves instantly). Player B notices it only 12.5 ms after it happened on server so he can quickly react.

We can add that it's not 12.5 ms + 75 ms after A pressed it's key. It's 75 ms+"delay between packet reception and next frame calculation"+12.5 ms.

At 40 fps (QL's servers standard. It can vary on Q3. Just do 1000/sv_fps), you get a new frame every 25 ms. That's the server's "built-in" lag. So the information could be received by B between 87.5 ms and 112,5 ms.
You must also add :
- The delay between the keypress and the next packet sent by A. I think it is done every time a frame is rendered unless cl_maxpackets isn't the same as the framerate. I'm not sure how it's done in that case.
- The delay between the packet reception by B and his next frame rendition.

- The delay between the keypress and the next packet sent by A. I think it is done every time a frame is rendered unless cl_maxpackets isn't the same as the framerate. I'm not sure how it's done in that case.

No it's done "snaps" value per second and no more than server fps. this isn't writeable cvar anymore since sv_fps got standarized in ql and every client is runing on snaps 40.

To clarify what has already been said:
- the server notifies all clients of location/health changes
- clients can simply ask to move/fire, and the server will do what it is asked when it gets that information (assuming still valid)

So the person disadvantaged is only the high ping player because it takes a while for them to find out about everyone else's changes, and it takes a while for changes they try to make to be applied.

Health / damage / collisions is always done by the server, so there is no issue with delay of that coming from clients.

The other minor mistake you made is that ping is the time for a round trip (to server and back), so that is why the other two replies deal with ping/2

Ahh duh. Dumb overlook on my part on ping. I see what you are saying though. Thanks all for clarifying.

Q4max was badass. You are a legend.