I found this thread (after I wrote the comment here:
https://board.jdownloader.org/showpo...2&postcount=16).
With this two new *.jar files in the \libs directory (not renamed!) the extraction of 7z files works fast again and with the same speed as *.rar or *.zip files.
My System:
Win10 64bit (build 1607, version 14393.479), fully updated
newest available JD, fully updated