Why this limitation on local network?
How can I change this to full speed?
Thank you for your help.

On the other hand, I really like this software, well done guys!

1. Are the files stored on HDDs? 2. What is the torrent piece size? 3. What is Settings > Transfers > Files > Peer read caching set to? I recommend 2MB blocks.

Unless you transfer from RAM Disk to RAM Disk or fast SSD to another fast SSD, you can't be sure the storage I/O isn't the limiting factor. Peak torrent transfers I've had from a single HDD were 15-20 MB/s too. That's because Bittorrent is supposed to download random pieces.
You can try to force a sequential download for LAN transfers. Please don't do that for internet transfers because this will likely kill a poorly seeded torrent.