Entropy in computers is the measure of the amount of randomness.
Low power consumption devices tend to work poorly as encryption generation devices because of low entropy. During testing I found that the standard /dev/urandom device scored 180-200 at idle. While this is good for low power use not so good if using a VPN or running stream encryption.
The solution is to use a different algorithm that takes more of the typically unassociated activity of the board into effect.
HAVEGED in the apt debian repo provides such a tool. This will give you by default an entropy of 4096. I am unsure if there is a correct ratio but I tend to think that at least the double of the size of the key is good enough. You will find that random number generation for larger string is much quicker.