There are 882,724 records in this Glamorgan Archives collection on Ancestry, from Bridgend, Cardiff, Merthyr Tydfil and Pontypridd. Checking in 10-year segments only 470,076 entries were found, 28% in the first decade of the 20th century.
UPDATE: The dataset is now renamed to replace Cardiff with Glamorgan. Ancestry may not always be sensitive to geographical niceties. In this instance they reacted quickly.