Java is designed so that you are forced to use threading everywhere. At the time it was written, exactly one flavour of Unix was designed to work well with heavy multi-threading, and that was Solaris. Competitors such as HP-UX used kernel threads and if you tried to open too many the box would slow to a crawl.

Sun has a long history of using its ability to control of Java certification as a bargaining chip in negotiations with vendors.

To get around the performance problems of Java, Sun has defined various extensions with efficient platform-specific external implementations. This includes things like vector math packages. In case the optimized implementation is not available for your platform, there is a fallback native Java implementation of the API which is much, much slower. Solaris has fast implementations of all of these standards. But other platforms, like Linux, have had trouble getting supprot even when companies existed that were ready, willing and able to pay necessary license fees and develop the package.

The result is that Java applications developed on Solaris that run fine on Windows will crawl to a halt on Linux.

Yes, Java may be write once, run everywhere. But not all platforms you run on are created equals...