Unfortunately, Bluetooth adds a significant delay to audio. This is unavoidable. It’s not a limitation of koala but part of the actual Bluetooth spec. There is no app on the play store that has good latency with Bluetooth, it’s a physical impossibility.
All software has this same latency problem with Bluetooth unfortunately and there’s nothing really to be done about it except wait for Bluetooth technology to advance to the point where it’s possible to have low latency bluetooth audio.
Here is an article to prove it’s not just koala but ALL apps.
There is nothing I can do beyond building a time machine, going into the future where civilization has cracked proper low-latency headphones, and bringing them back for you. Again, my knowledge of building time-machines is somewhat limited, so by the time I’ll have finished it, low latency bluetooth will have arrived anyway, so I’m not going to bother trying.