Agree, Bluetooth speakers will have too much latency using Bluetooth connection between the phone and speaker.
For further clarity, the above example I found between the different UE speaker products was after first disabling the Bluetooth connection to these speakers on my phone (only trying one speaker at a time)…Ie Settings—> Bluetooth —> select the speaker and Disconnect Bluetooth.
I then had a hard wire connection between iPhone and UE speaker, using the standard lightning-to-aux jack and cable.
Different speakers and different adapters provided different results.
hardwired to the following speakers.
UEBoom good result, a little latency added.
UEMega, poor latency (could depend on firmware of the speaker so try for yourself if you have one but way too much for my liking)
BoseS1 Pro, good result, little/no latency added.
Bose Home Audio, via front aux jack, OK but not the best, added a Noticable amount of latency but bearable.
Bose Noise cancelling headphones, great result, little/no added latency and great sounds you don’t hear otherwise, especially base (ie try with any good set of headphones and the experience is more realistic)
standard iPhone adapter OK, multi adapter NOT (as per photos above)
Hope this helps.
Encourage people if they find something useful to feed it back, but of course try different things for yourself.