In line 771 of phased.CustomAntennaElement there is a call to mag2db function within the saveObjectImpl method which is preceded by the following comment:
% starting from R2013a, we save linear scale pattern in
% pRadiationPattern to improve the performance, avoiding doing
% db conversion all the time. However, to avoid backwards
% compatibility issue, we will save pRadiationPattern always as
% dB and then convert it to linear scale when it gets loaded.
This desire to be backwards compatible results in a call that ends up being very expensive in terms of the time. It would be nice if we could avoid this.