Slow VPN connections on smartphone

Hi there,
I use a VPN to connect to my office network, over 4G, with the Strongswan app.
Here’s the weird thing:

  • connection speeds are great when I use my PC (PC → smartphone (USB Tether) → VPN through 4G → office server): ~60 Mbps
  • connection speeds are so slow they are barely usable when my smartphone is used directly (smartphone → VPN through 4G → office server): <1MBps
    So far I haven’t bee able to find something relevant on the Internet, any idea would be appreciated,