Agree absolutely with the above.

Unfortunate popular 'folk law' has it that higher speed cards will always perform better in a device. They won't for a device only reading from the card. So as well as all of upsss's very valid comments, particularly in regard to avoiding use of low-quality 'no-brand' cards, it's worthwhile to note that 'speed class' refers only to the card's relative write speed, the read speed will be the same for a Class 4 or 10 as he's noted anecdotally above, ditto for Class 2 the 'original' unspecified speed. However, even though Class 10 can write at ~2.5 times faster than Class 4 and 5 times faster than Class 2, because devices such as a GPS doesn't write to the card in normal operation the everyday performance will be the same. The card's wear-levelling characteristics which over time results in fragmentation of stored data will certainly slow down it's write speed but i'm not so sure even that will noticeably affect it's practical read speed in operation other than perhaps right at the very end of its write-cycle life.

Things are very different in devices which routinely write to the cards during normal operation, such as cameras. For instance, HD video cameras writing images to a card require very high speed cards [at least Class 10 or UHS-I] especially for recording fast-moving scenes. That's maybe the root of the incorrect folk law for other devices. Of course, having a faster writing card will allow the adding of data to it from a computer quicker provided it's in a high-speed reader. If the device's USB connection is USB1 for instance, a Class 10 card will under-perform greatly writing to it when inserted in the device compared to putting it in a USB3 reader plugged to a USB3 port. More info P#12/14 here: [Only registered and activated users can see links. ].