Data transfer over VPN slows down after 500 MB

[patcas]

Hello,
We are currently using a Site-to-Site OpenVPN (on a virtualized pfsense) to connect our datacenter (pfsense location) with our office (where our backup servers are located). Since we want to have off-site backups at our office (=remote location), we have to transfer a huge amount of data over the VPN. We then have experienced a weird thing:

) If we transfer the data using FTPS (over Internet, no VPN) to our Office, we get during the whole transfer a speed of 10 MB / Sec.
) If we transfer the data over the Site-to-Site VPN, the first 500 MB get transferred with 7.5 MB / sec and afterwards, the throughput drops to about 24KB / Sec. By pausing the transfer, the full speed is always available again after some minutes.

Does anybody know, what the limiting factor could be?

We would be very happy to be able to solve this issue, since transferring data over Site-to-Site VPN seems more secure to us.

Thank you very much!

Best regards
patcas

Our used modules on pfsense, which maybe play a role in this question:

• Snort Traffic Analyzer (Remote IP and Source IP not on block-list)
• OpenVPN with:
  Mode: Peer to Peer (Shared Key)
  Data Ciphers: AES-128-GCM, AES-256-CBC
  Digest: SHA256
  Data Encryption Negotiation: Enable Data Encryption Negotiation
  Hardware Crypto: No Hardware Crypto Acceleration
  Allow Compression: Decompress incoming, do not compress outgoing
  Compression: Disable Compression