In almost any game, players will have multiple copies of a card. The land/mana/energy cards in particular are usually repeated more than 4 times. The XML format, however, seems inefficient in that regard, especially when it comes to larger decks such as Magic's 5 color format.
Right now, the following code is repeated for each card you have in your main deck or sideboard:
<card>
<name id="234">Island</name>
<set>2011</set>
</card>
What I propose is an extra attribute, copies, that indicates how many times this card is used in the corresponding superzone.
<card copies="17">
<name id="234">Island</name>
<set>2011</set>
</card>
Hopefully this can be done in a future version. It may reduce load times, especially for bigger decks.
This will not reduce load times. 99% of the load time is from the images loading. The txt file parsing is really insignificant.
If you want a simple format, use the txt option.