Delay between pressing a key and the letter appearing on the screen

Sometimes actions that require disk access may experience a delay because the disk was put to sleep and it takes a few seconds to wake it up. The next time the same data must be loaded from disk the delay may not occur if the data has been cached in memory and physical access to the disk isn’t necessary.

That may or may not explain what you’re observing, but would explain why it happens some time after start up but neither immediately (because the disk has not been put to sleep yet) nor repeatedly (because the data on disk may have been cached in memory).

This could be a spell checking issue. Do you have a third-party spell checker installed? What are your Mail > Preferences > Composing > Check spelling settings? Try changing them to Never and see whether that makes a difference.

You’re welcome.

I get the delay even when clicking on 'Subject' after typing the address.
Difficult to see how a spell chancker could get in the way of that.

Not sure what you mean. If I check spelling as I type, Mail does certainly underline in red any misspelled words in the Subject...

Any chance the 'Slow Keys' button is selected (On) in Keyboard>Universal Access>System Preferences.

eMac 800 MHz, iMac Bondi Blue, iMac DV SE 400 MHz Graphite Mac OS X (10.4.8) iBook (Firewire) 466 MHz Graphite, iBook G4 1.33 MHz 14"