I will summarize the answers to this question as a FAQ in the wiki

What consideration, if any, should be given to card speed when buying new cards?

Does it make any difference to tablet performance for normal usage?

Video? Swap?

Any special kernels available that go faster using fast cards?

How fast should you go?

Will OS2008 make any difference?

I don't know but would like to... and since there has been all this talk lately about useful thread titles and whatnot I must say...

This is a very good question in a well titled thread...

Yes, there are numerous threads to sum up on this, going back to the 770.

"fanoush" is the poster to watch here.

Unless he says otherwise, I believe the main limiting factor here is less the card's intrinsic speed that the default (and prudent) timings used by Nokia for the driver in the kernel.

Even then, that is mostly important to those who want to boot their tablet from their memory card instead of the internal flash. In that case, not only is there much more space available for the system and apps, but it can boot and run quite faster using one of the special kernels with faster timings made available by fanoush and others.

The downside, of course, being that things may not work well with older or cheap cards that don't support those faster rates (defaults are there for a reason).

For the 770 the point is rather moot as the RS-MMC format is on the way out, tops out at 2GB capacity and probably won't go any faster that today. Fast kernels are available for OS2006 and OS2007HE, probably for OS2008HE if/when it comes out too.

Things are more interesting for the N800 as it uses standard SD and high-capacity SDHC cards, which are still evolving. Older cards used a "multiplier" speed index, much like CDs and DVDs : 80x, 100x, 150x etc. This has recently been replaced by the notion of "Class" (Class 4, 6, 8...) which is supposed to represent MB-per-second throughput.

However it seems that brand and quality are still important, as recent tests with the Asus EEE PC have shown that one brand's class 8 SDHC didn't show any performance improvement over another brand's Class 6 device. So I'd guess that a recent Class-something SDHC from a reputable maker with a current high-speed kernel would be a safe bet, with little reason to spend time and money looking for the top performer.

As for the N810, little information is available yet, both on OS2008 kernels and the (yet another) mini-SD format it uses, but maybe someone else knows...

Some follow ups:

So for stock kernels has Nokia tuned the i/o speed down so it does not really make a difference what speed the card is? Maybe it has been tuned to the speed of the on board flash.

If you go with a fast kernel then are there any implications for the (presumably slow?) on board flash memory? Maybe it is simply not used any more?

Referring to the SDHC cards (SD cards over 2GB) they are rated with 2, 4, and 6 classes, these represent the minimum write speed (in MB) if the card is empty, read speeds are normally higher than write. I have 2 8GB cards one is class 6 and the other is class 2 and I notice a difference when canola is scanning them. The class 2 card is really slow compared to the class 6.

In my N800 I don't run any swap. I did for a while but everything runs fine without one for me. I run the stock (low speed) kernel.

Now in my 770 it uses a swap but I have flashed the high speed kernel and notice a huge difference with the operation speed.

Both boot from internal flash.

Whether the card is empty or not should not make a difference as all current cards use sector spreading in their wear leveling, so that they are inherently physically fragmented.

However, the SDHC Class spec only guarantee a permanent minimum linear write speed for one large monolithic write operation, that being the critical factor in video use. Theoretically they can have any performance they want on multiple or random access operations, or when many small files are read or written in a fast sequence - which are typical computer use patterns. And given the history of benchmarks in the computer industry, we'd better expect less performance anywhere outside the benchmark targets - in particular on bargain cards where all efforts went into matching the one marketing-relevant "Class" benchmark, even at the price of degrading other performance critical parameters.

Yes, it's a sort of lowest common denominator.

On the 770 at least, booting from MMC with the stock kernel takes exactly the same time as from internal flash. I don't know whether this was on purpose though, or if it's also true of the N800 with SD/SDHC cards.

It's not used at the same time. The original purpose was either to have different versions of the IT OS in flash and on card, and boot from either as needed ; or to freeze a pristine, known-good image of a given version in flash, clone it to card, and use that afterwards for daily experiments (knowing that you can fall back on the original in case anything goes wrong...).

Let me try some use case questions along with tentative answers:

Does a high speed card make any difference:

Q. On a stock machine doing web browsing, regular apps?
- no

Q. When showing video?
- no

Q. When copying files from a host PC over the USB connection?
- yes, but within limits

Q. When copying files directly to the card inserted in a USB reader (ie. no IT involved) ?
- I suspect that this is the big win - especially if you want to copy big files (movies) to your card.

Q. On a tuned kernel booted from a high speed card?
- Yes, worth having if you are happy to fly this way.

Please confirm or deny my answers.

What is the speed or class of the cards shipped by Nokia with the device?

Should this then be the speed or class of larger, replacement cards?

The wiki page states:

It is not clear to me if this is an hardware or software limitation and how a patched kernel can affect this limitation.