It is not because class 2 or 4 is faster than class 10. The test spec for these classes are using large blocks of write to see how the card would handle long write for video and large photo files.
Once upon a time, many card designer optimize their speed by having the garbage collections done in the very last priority. The result is "hiccup" when you want it the worst, like video recording or large photo saving.
So the camera and camcorder producers get together for a spec to avoid these issues, by testing against large files (1MB to 4MB, for example) for worst case performance.
It is not about small files and it will never be an indicator for such. The reason of the false impression that class 2 or 4 will be faster for small file is because.... they are old cards with older memory (i.e. 56nm MLC instead of 32nm or 29nm 3LC). They were more expensive to produce but is now selling for a lower cost because they are no longer competitive, and the newer cards aren't even that big anymore.
Every generation of NAND have smaller cell size, larger block and page size, and today even more bit density per cell (3LC has 8 voltage levels instead of MLC's 4 voltage levels, or SLC's 2 voltage levle) and it takes even longer to write the same unit and they are even less durable. In flash memory you have to write one page at a time and erase one block at a time, so a lot of 4KB write will be easier to deal with in older memory with 2KB page and 512KB block than the newer memory of 4-8KB page and 2MB block.
My understanding, is that the lowest end card of SanDisk and the companies relabeling SanDisk, uses 3LC for its lower cost, then the "Ultra and low end Extreme III" uses MLC, and the Extreme IV and lightning series uses SLC. Withing this brand it would be a better indicator than speed class for small file speed, but i don't know how to compare across brand other than to avoid the lowest end cards.
Or you can dumpster dive for a small, old cards hoping for a fast one.