Thanks, nuknik, for your very usefull message.
My speed was 2 MBytes/s on my 1 Gbit/s ethernet network.
After desactivating IPV6 in W10, it runs at 110 MBytes
Asrock J3455 ITX, DS3615 version 6.1.7 (June 1.02b)
- Outcome of the update: SUCCESSFUL
- DSM version prior to update: DSM 6.2.1-23824 update 6
- Loader version and model: Jun v1.04b - DS918+
- Using custom extra.lzma: NO
- Installation type: BAREMETAL - ASRock J3455-ITX
- Additional comment: However, this update didn't solve my problem (reboot and shutdown not working).
So I downgraded to DS3615, 6.1.7, with loader 1.02b and now it works fine☺️