I remember you could do this in Amiga version, havent figured out how to do it in this remastered version yet.

You can set the text scroll speed in the gameplay section of the settings. You can also hit the spacebar during combat to immediately advance to the next message. In the original games you used to be able to use the arrow keys during combat to change the scroll speed; I haven't tried in the remaster, but I don't think that capability is implemented.

It's comma (less-than == slower) and period (greater-than == faster).

On what keyboard? An english one?

Yes, a US English one.

I always thought it was the < and > keys, and used shift for them.

Some ports used the cursor keys and some used other keys. This one copied the amiga with </, and >/. One of the best parts about the Amiga version of the game was the ability to right click and just drag a slider to set the combat speed. We have that functionality here but it isn't clear how it interacts with the keyboard speed change, because the keyboard speed change does not adjust the gui slider. Maybe they are combined?