Why hasn't China grown more rapidly?

Yes even since the boom it has underperformed relative to Japan, Taiwan, and South Korea. Ben Muse cites Martin Wolf:

The key to the puzzle, Wolf says, is, "inefficiency of investment." 

He points out that, among other things, the ratio of investment to additional output is high (that's bad, because "the lower [the ratio] the greater the bang for the investment buck."  China's five year moving average of this ratio is now 5; Japan, Taiwan, and S. Korea all had lower ratios.   The volume of bad loans by Chinese banks is also very high (in the Foreign Policy piece he notes that 40% of loans made by Chinese banks are bad).

"The principal explanation for the high level of losses [and I assume he means for the slow growth in general - Ben] has been the pouring of credit into the voracious maw of the state-owned enterprises..."

This is interesting:

"Still more remarkably, inward foreign direct investment was only about 4 per cent of GDP in the first half of the 1990s and 5 per cent in the second. Yet, according to two other IMF economists, FDI generated almost all of the efficiency gains. The share of foreign-owned companies in gross exports is also close to 50 per cent. Their share in the gross output of industrial enterprises rose from nothing in the early 1980s to 12 per cent in 1995 and 29 per cent in 2002."