Since there are problem when your RouterOS is configured as VPN client to the remote VPN server, you need to disable the FastTrack function in the firewall filter section. The maximum speed for single TCP stream will be reduced by half. In order to overcome this issue, you need to add FastTrack exception rule.
The following link(s) to give me some idea to configure. You may get more using the key words "RouterOS FastTrack and VPN"