Performance when there are *MANY* (> 30000) messages in account?

I am considering to migrate a certain account from another client to emClient.

One important issues is the number of messages in this old, long existing account. There are about 40000 messages. As far as I can see all messages in emClient are stored in one single file:

mail_data.dat

This could be a performance bottleneck for such a big amount of messages.

Does anyone have experiences with such a number of messages?

How about the startup time for emClient in such a scenario?

Does emClient read the full mail_data.dat at startup or only if necessary?

Peter