Troubled loans to borrowers outside Australia are curing at a much slower rate than loans to domestic customers.Quarterly data from the Australian Prudential Regulation Authority shows banks were holding A$6.7 billion in impaired loans outside Australia at December 2013. This metric peaked at $7.3 billion in March 2010.Given its track record, and comments made by National Australia Bank at its quarterly update, a large share of these loans are likely to belong to NAB and its troubled UK commercial loan book.Impaired loans to domestic borrowers have fallen by a third over the same period to $22 billion.The APRA data otherwise shows the Australian banking sector is in rude health, driven by the profits of the major banks.Return on equity increased two percentage points over the year to December 2013, to 14.2 per cent. The industry's return on assets increased to 0.8 per cent from 0.7 per cent over 2013.