As with other lines, signals failures, train fleet problems and action by customers were the big factors.
Transport for London measures delays in what it calls Lost Customer Hours (LCH). These are calculated by multiplying the delays in minutes by the number of passengers. TfL records all delays over 2 minutes. It uses the financial year from April to March and splits the year in 13 equal periods for performance measurement.
Data in the London Underground Performance Almanac for the last full year shows there was an average of 165,973 Lost Customer Hours per period on the lines. This makes it middle ranking in terms of performance across the network.
Over the past 10 years as Lost Customer Hours have been reduced across the network the Circle and Hammersmith and City lines have shown one of the biggest improvements with a 63% reduction in LCH comparing the average for 2003/06 with 2012/15
Since 2009 the Circle and the Hammersmith and City line have operated as a end-to-end service, perhaps more a spiral than a circle running from Hammersmith to Edgware Road via Aldgate.