Slow sendmail performance (javamail) with different mail api jars

I don't believe there is just one reason to make it slower. You may need to change some defaults just below example.

In the 1.6 document of the mail api for property mail.mime.address.usecanonicalhostname it states:

Use the InetAddress.getCanonicalHostName method to determine the host name in the InternetAddress.getLocalAddress method. With some network configurations, InetAddress.getCanonicalHostName may be slow or may return an address instead of a host name. In that case, setting this System property to false will cause the InetAddress.getHostName method to be used instead. The default is true.

So if you change the default value with false, it may changes its performance.

I would also recommend that check the java mail apis release notes to find out some clue.