Slow Login when using Proxy Server

We have a huge discrepancy in time to log in to our main server when using a proxy server compared to using the direct connection. This ends up with many users switching to direct connection, which we want to avoid at all costs.

Today the difference was as follows:

Log in via direct connection from Romania to Germany: ~ 14 seconds

Log in via proxy server from Romania to Germany: ~ 2 minutes 20 seconds

The same is happening across the other 9 proxy servers we use.

We are currently using Integrity 10.2. The connection to the main server is SSL, to the proxies it is clear.

Does anyone have any ideas why this should be happening? And if so, is there any way to improve this situation?

Thanks in advance.

Best regards,

Paul