Yellow bars mean the server is sending you data faster than you can receive it. Red lines indicate that you are losing packets. This is sometimes unavoidable, but if you are losing packets, you are certainly lagging. If you receive blue lines, something is wrong. Either the server just plain sucks, or your ISP does.
Bad packets cause the game to freeze solid.
Click to enlarge When packetloss is as bad as shown above, the game will not function properly or possibly at all. You should either quit and rejoin or find another server. There are a few ways to avoid lag in general. I know these are some of the best levels, but don't complain that your game is lagging if you know the cause. I still play on these maps all the time, but my ping is usually about 50 to 100 ms higher.