The current MethodTypeDesc implementation has a few performance issues. Below are the actionable changes that I recommend:

1. MTD can cache the descriptorString as computation is expensive; and its descriptorString computation can be improved by removing streams and formatting.
2. MTD doesn't have to validate all its arguments (to be non-void) every time a new instance is created, especially for return type swapping and parameter dropping.
3. Since parameterList will be costly, we should replace a few of its occurrences in performance-sensitive area with manually-unrolled loops.

This will most likely replace bug 8306889.

Also a few general side notes:
1. MTD parsing from descriptor is costly, so we can construct from CDs instead, especially if we don't use the descriptor.
2. As a conclusion, MTD should be stored in static final fields in use sites (like constants), such as in SystemModulesPlugin; this will be done in a separate patch.