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.
FIX 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.