Latency issues with APTX-LL receiver/transmitter used with video


Jul 3, 2021
I need to wirelessly send audio from a Roku stick in a projector on the ceiling to my soundbar approximately 15 feet away. There is no way to neatly run a physical cable between them, so I set up a bluetooth link between the projector and soundbar. However, I am having noticeable latency issues, meaning that I can tell there is a delay between the action on the screen (for example the actor's mouth moving) and the audio that I hear.

The soundbar is a Polk Audio Signa S2 (great sounding system for the money by the way), and it has an integrated bluetooth receiver along with an optical input.
The bluetooth transmiter is a Sonru (some brand that you only see in places like Amazon) transmitter/receiver which supports multiple bluetooth codecs including APTX-LL, the low-latency variant intended for use with video. The unit has a LCD screen that lets you know what codec it is actually using.

The audio path was: Roku > HDMI ARC audio extractor > Optical cable > Bluetooth transmitter > Soundbar (built-in bluetooth receiver).
This setup worked, however I was unhappy with the latency (audio delay). The LCD display on the transmitter showed that it was connecting to the soundbar via SBC (the most basic plain-vanilla variant of bluetooth).

So, I decided to buy another identical bluetooth unit (configured as a receiver) to connect to the soundbar's optical input, this way I would be able to use APTX-LL and enjoy the lower latency.
The new audio path was: Roku > HDMI ARC audio extractor > Optical cable > Bluetooth transmitter > Bluetooth receiver > Optical cable > Soundbar.
The bluetooth transmitter and receiver showed that it was now using the APTX-LL codec.

The audio quality was noticeably better with the new APTX-LL bluetooth receiver instead of the soundbar's built-in bluetooth. However, to my surprise the latency was even worse than before! I'd estimate that the delay is around a third of a second to a half a second (300ms - 500ms), wheras APTX-LL supposedly has latency of only 30ms.

Do you think that my bluetooth units are slow for some reason? And perhaps they have a lot of latency even though they are supposedly using the low-latency codec? Like maybe they are slow to get the data in or out of the optical connections?

Has anybody experienced latency issues like this before with APTX-LL transmitter/receiver pairs?
Has anybody tried a APTX-LL transmitter/receiver pair that worked well for you with no noticeable delay (when used with video)?