I built the source (latest one from github).
BTW, it seems to be due to the overheating of my device 0, which is too closely located to device 1.
DataParallel makes the device 1 wait for the device 0 of which power is reduced.
When I test the device 1 only, it seems to be faster than 1080ti single.
I have no idea how to handle this overheating. 
Thank you!