When using a Z Acme screw with 400 steps/mm the Z movement is extremely slow. Initially I made a mistake and did set it to 1600 steps/mm and then it works at speeds I am used to see (I have another printer working with M8 screws for years now). This behaviour is repeatable, whenever I set it back to 1600 steps/mm z speed is normal.
Changing max z speed, accelerations and othe suggested changes or using development version makes no noticeable difference (and yes - I have changed eeprom parameter between changes).
I can find no reason why firmware set much lower speed at 400 steps/mm compared to 1600 steps/mm. To me it would be natural to get same speed for whatever number of steps/mm while now there seems to be some type of set time involved to comlete a movement.
This item has been discussed several times before but none of the suggestions/explanations I have found have improved or explained the extremely slow z speed. I am sure a change to an M8 standard screw would speed up things but the preloaded T8 screw should be a technically better solution I would like to keep.