... it seems the option is hard-coded into gvfs.
I don't know what to do next.
You've answered your own question here. Either patch your gvfs to revert that change, or don't use gvfs.
]]>/dev/sde1 /run/media/gnucco/CHIAVINA16G vfat rw,nosuid,nodev,relatime,uid=1000,gid=1000,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,showexec,utf8,flush,errors=remount-ro 0 0
I tried copying the same file from windows 10 and it went ok, with a speed around 4 mb/s (not much but it's an old usb 2 stick).
Long story short, back under Arch I found out that umounting the automounted stick and then remounting it manually with almost the same options used by gvfs, but without the `flush` option, the file got copied without problems at the same speed I saw under windows. I tested this with other big files, manually mounting the stick with and without the `flush` option, and got consistent results.
Searching for a way to set gvfs automount to not use the `flush` option I came to this, so it seems the option is hard-coded into gvfs.
I don't know what to do next.