English is not my mother tongue. The question is whether the phrase "half as fast" is grammatical, frequent and unambiguous.
When googling, I've found out that the structure "verb + half as fast as" is frequent. But the search results for "verb + half as fast" are not convincing.
In January the growth of corporate loans was __%. In December the growth of corporate loans was half as fast, and the corporate loans in total increased by 5.6%. (by means addition or subtraction, half as fast means twice as slow).
Half as fast is relating to the growth (the rate of the increase). The percentage value 5.6 is relating to the increase in the corporate loans. The increase is equal to 5.6%. The rate of the increase in December is half as fast as the value measured in January.