There's a lot more to it than just the "Class." Flash memory devices have a finite write count. Meaning each Byte wears out after about 1000 or so writes. Reads do not really count. The Flash memory devices have an inbuilt operating system, usually cut-down Linux (SoC) that handles what is called "wear leveling." This does not instantly reuse previously freed up memory Bytes, it uses ones that have not been used at all or were freed a long time back. In fact Flash devices have more memory than they show as it is a buffer to be used to help extend the useful life fo the Flashdevice. For instance a 32GB SD card might actually have 40GB allowing the spare 8GB to be used in the wear-leveling over it's life.
As the Flash devices gets older, it takes longer to jump all over the place to write new stuff back. So, it is a good idea to replace Flash devices periodically if they are getting slower than they used to be.
OK, back to "Class." The Class does define Read/Write speeds but the increments between Classes were not all that great. The newer approach is "UC" ratings and the increase in speed is significant. So, get yourself as big (in GB) a card as you can afford with the highest UC rating, they will almost certainly all be Class-10. Not all devices (phone, tablet etc) handle the latest and greatest Flash memory protocols, so be aware that an expensive 128G, Class-10, UC-2 card costing a hundred bucks may not do any more than a 16GB Class-4 in your device. Just saying, ya know.
Having said that I have several 64GB Sandisk Extreme micro-SD ($60) for my Miix Tablet that are amazingly fast.
Caveat: Nope, I have nothing to do with Sandisk, I just like their stuff.
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