tl;dr: I tested an A2 'Application Performance' class card and found that it was half as fast as it should be, based on the SD Association specs (minimum 4000 IOPS 4K random read, 2000 IOPS write).
Then some people dug deeper and found in the specs that 'Command Queue and Cache functions' needed to be supported in device firmware and/or on the kernel level for A2 specs to be reached—though I haven't yet found a way for any consumer to get their hands on devices or software with this support... so no way to achieve claimed A2 performance.
So then I bought an A1 card from the same manufacturer (SanDisk), and it is actually much faster than the A2 card (though for price/performance you're still better off buying the not-Application-Performance-class-rated Samsung Evo+ card).
And in the end, being able to use an SSD or mSATA drive would be even better (the former of which is semi-possible today with the Pi 4 and USB 3.0—though you can't fully boot without a microSD card yet).
Then some people dug deeper and found in the specs that 'Command Queue and Cache functions' needed to be supported in device firmware and/or on the kernel level for A2 specs to be reached—though I haven't yet found a way for any consumer to get their hands on devices or software with this support... so no way to achieve claimed A2 performance.
So then I bought an A1 card from the same manufacturer (SanDisk), and it is actually much faster than the A2 card (though for price/performance you're still better off buying the not-Application-Performance-class-rated Samsung Evo+ card).
And in the end, being able to use an SSD or mSATA drive would be even better (the former of which is semi-possible today with the Pi 4 and USB 3.0—though you can't fully boot without a microSD card yet).