Actually, there is no direct relationship between an economy's rate of growth and where investors choose to place their money. The dollar fell by more than 10 percent in the period from 1977 to 1979, a period in which growth averaged 4.1 percent annually. It fell by close to 30 percent from 1985 to 1988, a period in which growth averaged 3.5 percent. From 1993 to 1995 the dollar fell by close to 10 percent, as growth averaged 3.0 percent. In each of these cases, the growth rate in the United States was the same or higher than that of its major trading partners, yet its currency suffered a substantial decline.
Investors care about getting the highest return on their money.
There is no direct relationship between an economy's growth rate and the return provided to investors. This article includes quotes from three sources, all of whom were employed by financial corporations in Europe. It is not clear that Japan's financial situation is especially precarious at all just now, nor that this move would be a reasonable response if it were. The market interest rate on Japan's long-term government debt is hovering near 2.0 percent. By comparison, the interest rate of U.S. government bonds is over 6.0 percent.