The two “2x” text objects didn’t have collision avoidance deactivated, so they were causing the extra vertical space between staves on those systems.
In Engrave mode, if you choose Staff Spacing in the toolbox on the left, you can see the frame fullness indicator - in your project, that was showing the first page was 114% full. On very full pages, Dorico’s spacing won’t be quite as good because things are stretched. So, deactivating collision avoidance for those 2x text objects helps a lot, as does flipping text objects properly (like the “Ausatmen” at the beginning - if you want text to appear below the staff, change its placement first, then adjust its position from there - otherwise Dorico allocates vertical space on the assumption that the text “should” be above the staff, whereas perhaps it actually fits better below).
If you don’t need the Copyright text frame at the bottom of the first page, delete it and use the space for the music frame to give yourself extra height on that page.
Here’s your project back with those changes, plus I decreased the music frame and page margins at the top and bottom a bit. I also assigned the “Silence” playback template to make it smaller when attaching here on the forum.
Sprechchor_LH.dorico (407.1 KB)
(Edit: and actually, putting the last system on the first page onto the second page produces what looks to me like a more balanced result - so bar 17 is at the top of p2)