Decreased performance when compiling with no options...

One of the things that we've done in Whidbey is add some extra IL instructions to improve the debugging experience. This allows you to (for example) set a break on a closing brace.

Because /o- is the default setting, this means that performance if you just compile with “csc” will be slightly degraded in Whidbey. If you've been building without setting /o+ and you care about perf, you will want to change your code to throw /o+ explicitly.

If you're using VS release and debug configurations, this will be set automatically for you.