Using a VPS as a VPN, I'm able to achieve 250mbps on a speedtest. However, when directly uploading files to the VPS, the upload speed drops to 80mbps. How does this makes sense?

Basically title. I'm using wireguard to set up the VPN.

Here's another similar situation on another set of VPS.

client -> Storage VPS (upload speed - 200kbps) client -> VPS1 as VPN -> Storage VPS (upload speed - 200kbps) client -> VPS1 with Storage VPS drive mounted on it (upload speed - 250mbps)