On my local environment (docker nginx + odoo + postgres) there is no issue with download speeds downloads is instant.
However when the application is hosted online at azure the download speed is about 1Mbit/s
Have performed a a peformed with a 24MB file on both odoo10 and odoo12 but the speed is the same.
What is this related to, or is it odoo that is this slow.
Maybe there is something bad with our nginx config.
The upload is super fast 300Mbits/s and it's just waits a bit for it to write.
What downloads speeds do you have downloading from ir.attachment?
Did you verify how long it takes to download from https://runbot.odoo.com/ ? So you can rule out configuration issues? It takes about 4 seconds for me for 22MB.