When I try to upload/download via Nextcloud or FTP in LAN - speed is about 110 MiB/s (max 1gbit eth).
I try to access from internet(WAN) throught my router - speed is about 2.8 MiB/s. But i use VPN client to my LAN from internet - speed is about 7.5 MiB/s.
I think - my router is cutting speed WAN-DietPI, but no. I install FTP server on my PC in LAN and create port forward in my router (the same i did for DietPI). And checked FTP speed from internet(WAN) throught my router to LAN PC. Is was about 8 MiB/s.
Could be some throttle from your ISP. If I understood correctly the problem occurs when you download directly from the internet. But when you download through a VPN connection to the home network, it works as expected.
If is it throttle from ISP - then speed FTP server on PC also will be slow, but it is about 60+.
Slowdown is only with dietpi linux device access from internet.
This is getting really strange. In your case, the connection between router and DietPi are always same. The only thing changing is the way on how to connect to your router and the router doing port forwarding. Correct?
Unless you have applied some throttling to the ftp server or the router, there is no explanation why the ftp would perform worse in the case of being directly accessed from the internet compared to VPN. However check the cpu utilization of dietpi just in case. I would also test with another router only the non-VPN solution.