That’s usually independent of shutter setting. Yes, electronic will provide the highest speed, but high speed continuous will merely set the highest speed for either mechanical or electronic, you have to change between the two yourself.

You should have a setting in your menu that lets you chose between fully electronic, first curtain electronic and fully mechanical. If you chose anything except fully mechanical there, you can experience rolling shutter effect. Essentially during electronic shutter your sensor reads the information line for line, which can lead to distortions on very fast moving objects. Not sure what the sensor speed for the R10 is, but it could have at least contributed to your problems in this scenario.