I’m still not sure I get it.

If, for the sake of argument, DHL takes 2 days to send a letter from Aus to the UK, how long would be a reasonable time for the postal system to do it?

4 days? 5?

Surely not twice that?

You get what you pay for. A DHL letter from NL to DE costs roughly 25 euros, a similar letter by mail at most 1,50 in stamps. No doubt for Australia the price difference will be likewise.