Investigate why CompileTheWorld takes so long with TieredCompilation

The ctw_2 test group runs CtwRunner on the java.base and java.desktop modules. The test group is run in two configurations: with, and without TieredCompilation. Without tiered (-XX:-TieredCompilation) the tests take a handful of minutes to run. With tiered (-XX:+TieredCompilation) the tests take 50+ minutes. While it is expected that the latter takes longer, it is not expected to take quite that long. It would be useful to know if why, and what can potentially done to improve on it.