I have an older manjaro installation media - I think from 2019.
My Internet speed isn’t super fast.
Of course Manjaro is a rolling release, so even if I install first from an older media, it will want to update the packages, therefore it would still want to download a lot of updates.
I am wondering - is it better to download a fresh copy of the installation image or will using one from 2019 be a faster installation?
There were a lot of changes since 2019. Better use a current boot disk or use the Architect Version.
The installation with a ISO from 2019 will be the same speed as a current one, but you will need to upgrade the whole system then what would be again 1-3 GiB traffic. Then a lot of configs changes since then. You will need to solve all old config files with pacdiff.