With the rise of actual computers that run from SD cards (like the Raspberry Pi) there has been an uptick in SD card performance as well as a new class rating system for cards that are designed to be used as a system drive.

Clearly crap slow SD cards still exist, but look for the new "A" rating for cards designed with being a system drive in mind.

Details here under Application Performance Class:

https://en.wikipedia.org/wiki/SD_card#Class

Do you think a A2 class card is a better option for a SD card drive?
I have test only two non-A-class cards, both with class 10 and UHS-I. Regarding the sequential read, the performance of fast cards will be restricted, right?!

Lexar SD card (class 10 and UHS-I), 300x, 45MB/s:

1. CrystalDiskMark test on USB 3.0 - sequential read: 44.33MB/s
2. SD to IDE adapter - sequential read: ~21 MB/s

Toshiba microSD-with SD adapter, (UHS-I):

1. CrystalDiskMark test on USB 3.0 - sequential read: 92MB/s
2. SD to IDE adapter - sequential read: ~21 MB/s

As wikipedia says, A2 requires host OS support for write caching and command queueing, so its likely A2 isnt the best idea. A1 doesnt need that.

Personally i'd look for an A1 card, that also has a decent sequential write rating, such as U3. This combination should hopefully get a card as fast as possible. However there will still be variations between cards. Theres a few sites around that are benchmarking card performance on RPi platform, so finding a card that performs well there will give best performance on a PC as well.

And yes, the old IDE interface and/or SD to IDE converter chipsets will limit the max throughput. Realistically though, its the random IO performance that you "feel" when using a PC, especially something retro, where the spinning disks of old were really slow anyway, and files are small so your never needing lots of sequential IO.

Sequential reads/writes arent even an indicator of random performance, and infact i've seen cards personally where they had slower sequential read/write, but much faster random IO.

I've tried both an A1 and A2 class Sandisk card with one of the SD to IDE converters - they both tested with the same results. Good enough burst speed and good random access (4MB read&write) for both. I saw an article on "A2" cards recently and why the speeds aren't that great - it supports command queueing but no SD adapter anywhere supports that feature right now, so the operations per second / random access are no better than "A1" cards.

The results for those cards were a little bit better than my Sandisk Extreme 8GB UDMA card (SanDisk SDCFX-008G-X46), but with much more storage and much more easily available.

These days I'm experimenting with SD as I've heard a lot of good from them, fixing lba chs translation, good dma support, speed on par with CF, etc. So my expectation where high. I wanted to replace a cheap 32gb CF I use to run win98. But I was very disappointed with SD, not only was it hard to make it boot, but then it was utterly slow. So I decided I would buy an fast sandisk extreme pro udma 7 32gb CF instead and do some more benchmarking with that card (thank you SpeedPAK, it wont take ages before it arrive)

What I found surprising is that, even the CF is equivalent or faster than a mechanical IDE, it actually feel way slower as win98 dont want to use DMA on it. The card say it support it on BIOS and tools such as NSSI. But win98 uncheck it on device manager every time. So the card seem like you are running in PIO and every disk access feel like a burden for the whole computer. It's not that bad and I prefer having a silent vintage computer than a fast one, but still I'm curious as to know why. Do you have an idea about that ? The cheap blue and white CF is the one pictured on my benchmark graphic.

Also the SD results are so bad (notice the lower graphic scale value is 10 time slower than the two other) it make me feel I did something wrong, I formatted it with partition magic 5 in a secondary ide as it seem harder to make it work than a CF when prepared from another computer. I intend to do so more experimentation tonight with stuff like minitool partition wizard.

I'm trying a different trail myself - SSD. Currently my 80486 is the first one to use an SSD in PIO-4 on a PTI-255W VLB controller through a SATA to IDE converter (Kingwin ADP-06). I have plans in the future to pick up an 8GB PATA SSD for my NEC Ultralight Versa Laptop (tried a 80GB EIDE Spindle in it with 95 for a hot minute, 40MHz on 8MB of RAM is just too slow for Windows 95 OSR 2.5 IMHO) for it's future WFWG/DOS 6.22 setup that I have planned for it (from the 258MB PATA Spindle that's in it now).

I've never tried the CF Card over IDE or SD Card setup, but recently getting a Rasperry Pi 4B from a co-worker has me toying with it a little bit since low capacity SD Cards are far cheaper than SSD. Might be something I'll look into later on.