First of all if you want to display text all the time - do not use Text object, use Spritefont instead. For the performance question... they are both the same probably.
The question is, do you need to show countdown text all the time?
Image below shows how I displayed 30 minutes countdown - just minutes without seconds, it looked liked this in game "30m", "17m" etc.
and i used Text object because this was shown only in two specific places in the game and it updates once a minute (and only while visible). For most of the game time this was destroyed. On game over screen it was created, time was compared and then displayed.