Complex sentence: The 1990s are often referred to as Japan's Lost Decade, and economic performance has frequently been poor in the following decades; the stock market never returned to its pre- 1989 highs.
Split into simpler sentences: The 1990s are often referred to as Japan's Lost Decade. However, economic performance has frequently been poor in the following decades; the stock market never returned to its pre- 1989 highs.