I re-tried my install instructions for both Ubuntu 18.04.4 and 19.10 64bit (instead of Raspbian 32-bit), for Mattermost Team server.
That’s the fix!! Don’t use Raspbian 32-bit. Use Ubuntu 18.04.4 64bit, or 19.10 64-bit.
There’s something about 32bit Raspbian which is slow, for Mattermost. But 64bit Ubuntu 18.04.4 (or 19.10) rips way, way faster on an image upload. The server is totally usable in a daily driver sort of way!!
How much faster?
Here’s a test I did. I ran Mattermost Team Server in a Debian 10.3 64-bit VM (ARM64). I gave the VM only one of my CPU cores, non-hyperthreaded (SMT is disabled). This is an i5 Intel Core, 8th gen. A laptop, Asus UX330U. Then I uploaded a 4.0 MB image. The thumbnail-processing with maxed-out CPU took just under 4 seconds (just timed on a wristwatch). Then I uploaded that same 4.0MB image into the RPi4. It also took just under 4 seconds (utilizing all 4 cores). Like it was identical performance! This is a night and day difference, going from 32-bit to 64-bit. This is going from minutes and minutes, down to under 4 seconds. That 4.0MB image would have crippled the RPi4 in 32bit Raspbian!
Problem solved! No code changes needed!