Easing Into Using the New AutoStats Feature - comment by carrie

Lots of things change when you go from 13.10 all the way to 15.0.  So there is no simple answer to any question involving a change in performance.
 
You did not say if the CDR data was uncompressed in 13.10 and then compressed only after you went to 15.0.  If that is the case (uncompressed before upgrade, compressed after), then you can expect more CPU to be required to collect statistics.   When data blocks of a table are compressed, they must be uncompressed before statistics can be collected.  That could cause the job to run longer.
 
Make sure you are collecting statistics with the new 14.0 sytax, where you bundle all the individual column/index collections into one collect statement, that way you will reduce the table scan effort, and therefore the decompression effort.
 
Check the actual CPU being used for the statistics collection before and after the upgrade.   It may be  somewhat greater post-upgrade, but should not be orders of magnitude greater.   If CPU usage to collect an individual statistic is in the same ballpark (maybe 10% or so greater than it was in 13.10, or if using BLC for the first time, maybe 30% to 50% greater), then that seems reasonable to me.  
 
If the CPU reported for the stats collection is acceptable, other things you could check are if the stats are running at a different (lower) priority, or is there more active work running on the platform leading to more contention for resources.
 
Thanks, -Carrie