Windows server 2019 RRAS slow download speed

Having issue with L2TP VPN on Windows Server, establishing home-to-org network. At home I got a default 100mb internet, so download speed is ok here. The symptoms are following:

1. Trying to upload 300MB file to network share from home PC (Win10), upload speed is ok, like 5Mb/s.
2. Same file downloading back – explorer shows calculating and cannot even start download
3. Trying to open small files (like docx 100Kb file) takes like 1 minute or so (shows downloading 0% on Word splash screen).
4. RDP through VPN works like ***. Huge delays, black squares, black screen after Alt-tab.
5. All things above work like a charm if trying to do them without VPN (eg. port forward RDP, SMB), same stands for PCs in internal network.
   Server is behind Mikrotik NAT, and is not on VM. How to sort this out?