Commonwealth Bank's performance was superior to its big bank rivals during the first half of the financial year, based on several key measures. Although not directly comparable (CBA's half-year ends on December 31, while for the other three banks it is March 31), CBA outperformed its rivals on profit growth, income growth, return on equity and margin growth.The most significant factor in the improved cash earnings for the Big Four banks in the first half of the financial year was a reduction in their charges for bad and doubtful debt.PricewaterhouseCoopers calculated that 60 per cent of the improvement in cash earnings in the half was driven by reduced bad debt charges.NAB put in the best bad debt performance. It impairment charge was half that of the previous corresponding period - down from A$1.2 billion in March last year to $528 million in the latest half.NAB chief executive Cameron Clyne said the big reduction was a result of the program to "de-risk" the bank's lending portfolios.Commonwealth Bank also reported a big reduction in bad debts, writing off $457 million in loan impairment expenses. This was down 26 per cent on the previous corresponding period.Westpac's charge of $341 million was down 22 per cent and ANZ's charge fell from $599 million in March last year to $528 million in the latest half - a reduction of 12 per cent.On the basis of its de-risking program, NAB increased its cash profit by 8.5 per cent over the previous corresponding period. In terms of profit growth, CBA had the best result, increasing its cash profit by 14 per cent. ANZ increased cash profit by 11 per cent and Westpac increased its cash profit by eight per cent.Looking at top line income growth, CBA's operating income was up eight per cent, ANZ's was up six per cent, Westpac's up five per cent and NAB's up 2.6 per cent.One of the weak spots was an increase in costs. KPMG said the big banks' cost-to-income ratios went up an average of 116 basis points.KPMG said: "Strategic initiatives to improve productivity, greater levels of offshoring, simplification of operating models and disciplined investment spending will continue to be priorities."Westpac's operating expenses rose six per cent over the previous corresponding period, while its cost-to-income ratio rose from 40.9 to 41.2 per cent. Despite the increase, Westpac has the lowest cost ratio among the big banks.CBA's operating expenses rose six per cent, while its cost-to-income ratio fell from 43.8 per cent to 42.9 per cent.ANZ's operating expenses rose six per cent, while its cost-to-income ratio fell two bps to 44.3 per cent.NAB's expenses rose 11.6 per cent, while its cost-to-income ratio rose from 41.6 per cent to 45.4 per cent.All commentators agreed that the banks would continue to face pressure on their margins, due to the combination of modest lending growth, intense competition for deposits and loans, and low interest rates. PwC said it was likely that margins would fall further.Commonwealth Bank was the only one of the four to report