Diagnosis
This is typically a software cache issue with webOS or insufficient RAM for the 4K stream. The TV's network hardware or software is struggling.
How to Fix It
Clear the app cache and data. Go to Settings > General > Storage > Internal Storage > select 'Cached Data' and clear it. Then go to Settings > Support > Software Update > check for updates. Install any available updates. After updating, power cycle the TV (Settings > General > turn off Quick Start+, then unplug for 2 minutes).
If That Didn't Work
Change the DNS server. Go to Settings > Network > Wi-Fi Connection (or Wired) > Advanced Settings > DNS Setting > set to Manual. Use 8.8.8.8 (Google) or 1.1.1.1 (Cloudflare). This bypasses often slower ISP DNS servers.
Heads Up
The 65NANO75UQA has a 10/100 Ethernet port, not Gigabit. For high-bitrate 4K streams, a strong 5GHz Wi-Fi connection may outperform the wired Ethernet. Avoid using VPNs on the router as webOS apps often fail with them.