If you use Teamviewer's device code it forces traffic through their infrastructure, thus slowing it down (it also allows them to snoop on your connection)
Have your end client set up SSL VPN on their firewall or their WAN router.
That client will then provide you with a VPN login. You'll have to install the VPN client (cisco, Sonicwall, Juniper, Fortinet, etc).
Use the client to establish a VPN tunnel directly to their network.
Once on their network, use the remote machine's IP address instead of it's TV code to connect directly to it.