Why is pptp speed slow?

Question

VPN

Hello.
Network #1 (192.168.1.0/24). There is access to the Internet, the provider is regional. Inside the provider, traffic is free, global is paid.
Network #2 (192.168.88.0/24). There is access to the Internet, the provider is regional. Inside the provider, traffic is free, global is paid. In addition, this network has unlimited internet through a second router (192.168.88.2). Also on the second router, a 1TB hard drive is installed for the media server and just file storage.
Through a regional provider, I connected them via PPTP (10.0.8.0/24) and passed Internet traffic through 192.168.88.2.
When you try to download from the Internet from network No. 1, the download speed easily reaches a maximum (10mb / s), but when you go to \\192.168.88.2 and try to download files from the router, the speed does not rise above 250kb / s.
If you download files from \\192.168.88.2 from network No. 2, then the download speed is 45MB / s.
Thus, we conclude that pptp does not cut the Internet channel (almost), but when you try to download files from network to network, the speed is cut very much.
What can be wrong?