In general, ARC is a bit of a disaster. Try a digital optical cable instead that to see if some of the lag is diminished.
Plan B would be to pitch the sound bar and obtain a very basic AV receiver with surround speakers/sub and option built in to adjust audio delay.
I purchased this Sony sondbar:
The audio has a 1-2 second lag which is driving me nuts. I have it connected directly to the HDMI 3 ARC slot. PS4 which I use for all games/streaming directly into HDMI 2.
Audio set to PCM.
I've tried every fix I could locate on Youtube/google and just can't seem to get this to sync up properly.
If it matters the model of the TV is:
Sony - 65" Class - LED - X750D Series - 2160p - Smart - 4K UHD TV with HDR
Any help would be GREATLY appreciated!!