We use the API in memory mode, and recently updated to the latest data file in our dev environment. This environment does not see a huge amount of traffic (400 requests per second) and previously had an average latency of less than 5 milliseconds.
Upon restarting the server with the new data file, average latency never went below 80 milliseconds, and often went over 1 second.
The new data file was the only change.
We updated to the latest version of the API (188.8.131.52), but that did not decrease the latency.
I'm unsure when we last updated the data file, but it's safe to say it was many months ago.