The only thing that has ANY meaning is the speed-class or UHS rating of the card.
Any other indication/name/label whatever is just marketing blurb and doesn't mean anything.
This is the small number with a capital C around it for older cards (speedclass) or the number with a capital U around it for newer cards (UHS).
Speedclass goes upto 10 and should be read as: ZZ Megabytes/second write speed.
UHS currently goes up to 3 (might go higher in the future) and should be read as: ZZ * 10 Megabytes/second.
E.g. UHS-3 is 30 MB/s and SC-4 is 4 MB/s.
The class indicates the MINIMUM write speed the card can handle. And nothing else.
The speed-rating is only valid for sustained write through-put with the FAT32 filesystem. Random-access behavior or performance when formatted using a different filesystem may vary wildly (and is usually worse).
Manufacturers usually source the components from various chip-makers and it is quite possible those chips don't perform the same.
2 identical cards can perform differently because there is no telling if they both use exactly the same innards.
But they should at least be able to do the minimum advertized speed as advertized by the class.
The form-factor (SD, Micro-SD, nano-SD, with or without adapter) doesn't say anything about the chip inside and has no bearing on the performance.
Typically: Cheap cards usually just make their speed-rating barely. It's a cut-throat market with low margins and every corner that can be cut will be cut.
There is a reason professional photographers/filmers tend to buy the expensive high-end cards. They usually perform better and are also less prone to bit-rot after prolonged use. And the more expensive stuff usually comes with a warranty that lasts longer than the time it takes the buyer to leave the shop with the card.
Thanks. The SD adapter doesn't cause any reduction in performance at all? (When I say "at all", I mean anything that a program like CrystalDiskMark can detect. It doesn't matter if a 60 minute operation is 1 nanosecond faster/slower overall, but a difference of even 1 second for a 60 minute test does matter.) – RockPaperLizard – 2014-12-01T09:46:23.793
Can you explain the vastly faster speeds of the microSD cards when compared to identically performance-spec'd full size SD cards (from the same manufacturer) as reported in the article in PhilippT's answer? I can't explain that one. Maybe you can... – RockPaperLizard – 2014-12-01T09:54:17.453
The SD adapter doesn't perform any signal processing, it's basically just electrical contacts. I don't know what to make of the table in that article and what testing it reflects. One difference: the cards are different capacities. I don't know if "Extreme" is a regulated term and whether it refers to a specific performance vs. a range of performance. The fact that one company offers two products labeled "Extreme" doesn't mean they have the same thing under the hood. They could have made a better MicroSD card than SD card. That doesn't necessarily relate to the form factor. – fixer1234 – 2014-12-01T10:09:19.783
There can be a lot of variation from card to card. My understanding is that they manufacture cards and then test them to see how fast they are, how much usable capacity they have, etc. Then they label them accordingly. SanDisk may have a MicroSD card they label "Extreme" and and SD card they label "Extreme", and that MicroSD card may be faster than that SD card. But that doesn't tell you whether one form factor is inherently faster than the other. The issue is really how do you find the fastest card in either format. – fixer1234 – 2014-12-01T10:18:41.927
Thanks for all the info. To my knowledge, there is zero regulation. Your hypothesis regarding identically marked cards (from the same manufacturer) having different speeds is interesting. I wonder if anyone has tested 10 identically marked cards from the same manufacturer to determine the variability. Most 'expert' testing on the internet ignores this hypothesis, however anecdotal reports do vary wildly. It would make an interesting report. – RockPaperLizard – 2014-12-01T10:28:27.620
3@RockPaperLizard: My previous company did. We did discover that two identically marked cards would often have different internals, especially those sold in the consumer market. SanDisk specifically was identified as a brand which will sell you catch-of-the-day. We ended up going with a supplier that guaranteed the chips used, but at a price twice that of consumer brands despite 100K unit volumes. – MSalters – 2014-12-01T12:06:52.397
@MSalters Great info, thanks. To be clear, your previous company tested identically marked cards from the same brand (e.g. SanDisk), and the cards often had different internal components? – RockPaperLizard – 2014-12-01T12:18:26.307
Well, we didn't buy enough cards in a statistically valid way to say that it was 34% +/- 2%, but it was both a practical outcome of checking and confirmed by purchasing (who apparently asked for a quotation of 100K identical SD cards, but couldn't get it) – MSalters – 2014-12-01T12:31:15.633
@MSalters Sorry, I wasn't clear. I want to understand if the cards tested weren't only identically spec'd, but also were from the same brand (e.g. SanDisk). – RockPaperLizard – 2014-12-01T20:49:59.133
@RockPaperLizard Re: Your question about the microSD -> SD adapters; if you want to take a look for yourself, find an adapter you aren't using and crack it open (you can usually just stick a flat thing in the microSD slot and pry it open). As you can see, it's just some metal contacts that directly connect the microSD pins to the SD pins. Or if you don't have one to spare, here is a picture.
– Jason C – 2014-12-02T01:12:05.737@RockPaperLizard: Yes they were. – MSalters – 2014-12-02T12:44:54.717
@fixer1234 Wow! This question just hit 10,000 views! Thanks again for your answer. Do you know if SD cards were always just microSD cards with long contacts attached, or at one time did they have larger non-volatile memory chips inside? – RockPaperLizard – 2016-06-02T20:58:45.793
@RockPaperLizard Hey, congrats on passing 10K views! I don't know for sure, but I'm guessing that SD cards came first and their size was based on holding the current technology. As they were able to shrink things more, that enabled the microSD format. But that's just a guess. – fixer1234 – 2016-06-02T21:25:25.367