Serial0 vs Serial1 speeds

Hi. Serial1 transfer speed is much slower than Serial0 on my PX2.1. I use APM Copter 3.5 and tried many different settings, no matter what I can not download dataflash logs via Mission Planner as fast as with Serial0. 30x difference.

Please see my post here: Serial0 vs Serial1 datarates

Thank you.
Roman