Choppy play with Plex+Chromecast?

Running TrueNAS-12.0-U2.1 with Plex media server and Chromecast Ultra connected to a wired 1Gbit Cat6 connection. There is no transcoding and I'm streaming a low 10Mbps (1MB/s), yet the video played on my TV is jittery, is like every few seconds there are few frames missing. The audio plays properly in sync. Playing the file directly from NAS on my Mac is perfect, no jittering,

Any idea how to fix this? It is extremely annoying and quite unwatchable...

Choppy play with Plex+Chromecast?