I just bought a Soundcore 2 Bluetooth Speaker. I need the aux port, which I am using. There is significant audio delay using the 3.5mm input. Around 200ms, as would be normal for bluetooth. Here is a sound file with the audio from the Soundcore 2 in one ear, and the audio from my old Anker speaker in the other:
https://1drv.ms/u/s!Av2PnyHhnkFIk8UadeJZvCFVM9Heow?e=IfTwB3
Which of the modern speakers have no lag when used with a 3.5mm analogue input?