Tbh, I only ever noticed that the Play Store didn't do parallel downloads like once a year when switching phones. I forgot about it the rest of the time lol
It'll improve the overall time. Pressing the download button doesn't saturate your downlink immediately, it's always a ramp up to max speed. Doing them in parallel saturates your connection much better.
Are you downloading from several mirrors at the same time? In that case it could be faster because the individual mirrors have a lower bandwidth than you have.
Wonder if google would do something like this, but I would guess they download everything from the same server local to you, so I don't think it should have any effect really.
It couldn't download while an app is installing. So while an already downloaded app was installing, you had nothing downloading until it finishes installing.