Performance problem with OpenVPN and L2TP connections

Dear all, i have a serious problem about performances with my network environment. I have this situation:

One central mikrotik (RB750 with firmware 5.21), say it Core, that operates as OpenVPN and L2TP Server.
Many periferical mikrotik (same model and firmware) in different countries, say them Clients, that operates as L2TP Clients.

The Clients are connected to the Core via L2TP; my users connect to the Core via OpenVPN.
Then, via software, they can choose to route via one Clients or another, so they can surf the web with the remote IP Address of the Clients.

With this configuration, the performances are very low, often my users are not able to connect to any web sites.
Instead, if they connect directly to the Clients via OpenVPN the performances are good enough.

On the core router, looking at the Rx Stats of the WAN Interface, I see a large number of fragment, more than 12000, and always growthing. Can this be the problem? Any ideas?

Thank you