In a comment to this Hardware Recommendations SE question, Agent_L writes:

There is no such thing as "fastest" card, eg cards with higher (like 10) class are faster for large files (eg video) but for small files class 4 cards are the faster ones.

Are Class 4 microSD cards faster than Class 10 microSD cards for small files?

Answers that include benchmark testing results will be most appreciated.

  • I cannot find evidence of what Agent_L wrote on the answer... Maybe he is wrong? sdcard.org/consumers/speed – xdola Dec 14 '15 at 20:34
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    I dont mean to be trite, but "some dude said so" is not a great place to start. have you found simmilar oppinions online or any indicate that these statements have an actual basis in fact? Class 4 cards can support up to 4MB/s and class 10 can support up to 10MB/s. sdcard-speed-guide.articles.r-tt.com – Frank Thomas Dec 14 '15 at 20:34
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    @FrankThomas I have never heard of that claim before, but I always start by assuming others know more than myself. This QA will be the perfect place for Agent_L to support his/her statement, and then either we will all learn from him/her, or the opposite. – RockPaperLz- Mask it or Casket Dec 14 '15 at 20:43
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    @Agent_L : So where is this information from? – paradroid Dec 14 '15 at 23:03

It mostly comes form here. Long story bit shorter (but still quite long): Back in the early days of Windows Phone 7, some devices had SD cards slot. It wasn't mean to be user accessible, it was meant only for manufacturers, to offer bit of flexibility for different markets. The phone at first boot detected, locked and encrypted the card. Since then the card wasn't just another kind of storage - it became a physical extension of system memory, indistinguishable and irremovable (removal of the card corrupted phone memory and required full reset). Such usage (as system drive) put unusual requirements on performance of the card. Any delays in random-access resulted in unacceptable lag for the entire phone. So, Microsoft tested many cards and found out that top class-rated cards don't perform well. The list of cards that did deliver best random-access performance unfortunately consisted of models not available in retail.

My statement in comment was clearly an oversimplification because I remembered it wrong. I should have been more clear:

SD card class rating measures only sequential performance (eg films). Higher class-rated card in random-access performance (eg apps) can be beaten by lower class-rated (or unrated) card, but it's not an absolute rule.

Only comparing cards of same era (and price range) makes sense. By no means I wanted to say that cheapest class-4 noname will beat most expensive class-10 in anything (although I did).

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