Why is there a time lag when I put my iPhone on loudspeaker?
The main technical problem with most speaker phones is that they are not full duplex.

This means, that unlike a normal phone conversation, both parties cannot speak at the same time. To avoid feedback, the system mutes the microphone most of the time and instead tries to detect when you are about to speak.

When both parties are on speaker phones, this system works quite badly since the normal conversation queues of "mmmm, uh-huh, go on..." can cause it to cut the real conversation off at times.

To fix this, don’t use speaker and use headset or Bluetooth for your mic.