OK, one more – this one’s good. Long story short, I ran
sudo tune2fs /dev/mmcblk1p7 -E stride=16,stripe_width=1024
(replace mmcblk1p7 as needed) and saw quite a bit more ~5MB a second writes and less of the like 32KB/sec or so writes. The ext2/3/4 filesystem tries to allocate new files within a SDCard write block and erase block with these options, instead of allocating them wherever. This should also greatly reduce card wear.
So, what’s this do? These settings are designed for RAID. The stride is the size of a basic RAID block, you write a 512-byte block and the RAID system must read it, modify it, write it back (and write some parity for RAID5.) Well, the SDCard has a minimum block size where it must do this too, this is typically 16-64KB, I choose 64KB. The stripe-width is based on stride * number of data disks, since you have x data + 1 or 2 parity disks keeping writes within a stripe speeds things up somewhat. On SDCard, you have an erase block/allocation unit size that is like 2-4MB where keeping writes within this block greatly improves write speed, I choose 4MB. When you run mke2fs (or mkfs.ext2/3/4) with the -E stride=x,stripe_width=y, it tries to make sure the metadata is spread between disks (probably useless for SDCard.) But you can turn it on even after the fact, and it should try to allocate new files so they are within write block/erase block boundaries on the SDCard. In practice it seems to help quite a bit.
This site has some info on how to determine your write block and erase block size, plus a table with a bunch of cards listed. I didn’t measure my card, the bigger cards were mostly 64KB write size and 4MB erase block so I picked that.