Strange case of delayed chat

I was talking to a player today, who said they thought their chat messages were delayed, so we both opened up a clock in a web browser to test it out. I was receiving their chat messages almost instantly, but it took about 5 seconds for messages I sent to be delivered to them.

I found this odd because we were both using the singapore server, and even a high ping can't account for 5 seconds (they said they normally have 50-100ms ping).

Just thought you guys might like to know about it.