There are a couple of things for improving SimpleDateFormat performance.

The 4326988 fix requires to scan the given pattern string to verify it. It's possible to compile a pattern string with a little extra of cpu cycles so that further scans (e.g., counting 'y's) of the same pattern in format() and parse().

SimpleDateFormat.format() invokes DecimalFormat to format numbers, such as a year number, in a locale-dependent way. However, numbers to be formatted are usually in 1, 2 or 4 digits. Some optimization can be done.