When looking at external API-driven charts of Mt Gox trades, there is a "Trading engine lag" shown, which shows the backlog of orders queued and waiting-to-be-executed, relative to the present time:
(For example, see http://bitcoinity.org/markets, and http://bitcoin.clarkmoody.com/ )
How is the "engine lag" time calculated?
It is the difference between current timestamp and the submission timestamp of the oldest order submitted, which has not yet been processed by the order book.
From the Mt. Gox API Documentation:
Order lag
https://data.mtgox.com/api/1/generic/order/lag
The "lag" value is the age in microseconds of the oldest order pending execution. If it's too large, it means the engine is busy, and the depth is probably not reliable
