Without proper measurement / data nobody can help you. Like:
- ping from your PC to 8.8.8.8 or 1.1.1.1 or some other fast sites
- ping from your friend’s PC ?
- ping between you too through ZT ?
I recommend to use 9ping, which helps to see clear data, running for 5-50 minutes.
Also maybe read my answer in the other topic:
But that costs money. (Self host a 1core VPS in your own country = $5 / month)
If ZT does not work for you, you may try other VPN systems, like:
WireGuard, SoftEtherVPN, etc…