Lag corresponding with 503's from master server. Is the request done in LUA?

Seeing a lot of

HTTP request dcs:checksession failed with error 28: Timeout was reached

at the same time we're seeing a good deal of lag on our server. Is the outgoing request done in LUA and single threaded/blocking until the timeout is reached?

Our networking is pretty ok at the time, and we're also seeing 503's which further lead me to believe that the problem isn't on our connection's end, but I could wrong about that.