It may not be the best choice but I selected my country, then auto.mirror.devuan.org and my download speed seems to be fine.

- pkgmaster.devuan.org
- auto.mirror.devuan.org

I don't remember exactly but I think we have to choose a location (country) before selecting the mirror. In which country(ies) are these 2 mirrors located?

There are two things that affect your speed of downloading packages, one is mirror's capability to serve files, the other is network distance.  Network distance may not be as relevant to geography as you think.  If you were to get a list of mirrors from different countries/states/territories etc. and write a little script pinging each one 3-4 times (ping -c 4 deb.debian.ca for example) you get a relative speed of the connection, but not the rate it can upload files to you.  Then run the same some hours later or days and you may get a different picture.  So don't be so concerned.  If you are in SW Australia I wouldn't pick Iceland, or from N. Korea I would not pick Johanesborg, but I have had a mirror within a bicycle's ride away be three times as slow as one three continents away.

Then there are those clueless people using tor and one moment they seem as being in N.Korea the next moment they are in Cambodia.

Those clueless people were shoved without notice in an experimental repository system without notice and without a way to be able to tell.  But they don't worry about speed anyway.

For jessie, auto.mirror.devuan.org is fine.

auto.mirror.devuan.org then.

I am downloading the following iso:
devuan_jessie_1.0.0_i386_DVD.iso                   24-May-2017 00:10      4G
https://files.devuan.org/devuan_jessie/installer-iso/

so yes, I use the Debian installer. I should easily find auto.mirror.devuan.org though.

Last time I chose a mirror during installation

When I install Devuan (during the installation, not before), I am prompted to choose a mirror to get additional packages. So you advise me to use:

- pkgmaster.devuan.org
- auto.mirror.devuan.org

Use one or the other.  For jessie, auto.mirror.devuan.org is fine.  For more complete mirrors (including the file descriptions) use pkgmaster.  Note that the mirrors are not yet integrated into pkgmaster.

Sorry, can't help you there.  Haven't installed using the Debian installer in a very long time.  I'm guessing the default would still be auto.mirror.

Last time I chose a mirror during installation

When I install Devuan (during the installation, not before), I am prompted to choose a mirror to get additional packages. So you advise me to use:

- pkgmaster.devuan.org
- auto.mirror.devuan.org

I don't remember exactly but I think we have to choose a location (country) before selecting the mirror. In which country(ies) are these 2 mirrors located?

I need to install Devuan on a new PC today and I don't know which mirror to choose. Last time I chose a mirror during installation (the closest to my location), a few packages did not work properly so I used the packages included in the DVD.iso instead.

- Do you recommend the DVD.iso without the mirrors for maximum stability?
- If I want to use a mirror to get more packages, which one should I choose? Is there an official one?

Thanks!