For 2014 (and only 2014, as the Spanish government realized they had done really stupid), short term capital gains (ie 1 year or less) are taxed at your general income marginal rates (up to 52% yeah!) instead of as part of your savings base (which is taxed separately around 21%).
Normally, when you do your US foreign tax credit, you calculate it separately in two buckets: first for your general income (salary etc), then for passive income (interest, dividends, capital gains, etc). (There are a couple more special buckets).
Now comes the annoying part: any passive income that is taxed at a rate higher than the highest US marginal rate (39.6%) gets “kicked out” of the passive bucket and put into the general bucket. The reason this is annoying is that you cannot use buckets to offset against each other. You overpay your capital gains at 52%, but underpay your interest (21%), but you can’t apply one against the other.
Luckily this is going away next year.