jfrEvents.hpp depends on many header files. It's included (mostly indirectly) by 231 out of about 966 .o files:

$ find hotspot/ -name \*.d -type f | wc
    966 966 49653
$ find hotspot/ -name \*.d -type f | xargs cat | wc
 550940 1099952 39642808
$ find hotspot/ -name \*.d -type f | xargs grep -l jfrEvents.hpp | wc
    231 231 11667

Most of the dependencies are caused by these 3 header files, which just has minor dependencies in JFR

http://hg.openjdk.java.net/jdk/jdk/file/0348e9be94d1/src/hotspot/share/opto/compile.hpp
http://hg.openjdk.java.net/jdk/jdk/file/0348e9be94d1/src/hotspot/share/compiler/compilerEvent.hpp
http://hg.openjdk.java.net/jdk/jdk/file/0348e9be94d1/src/hotspot/share/gc/g1/g1GCPhaseTimes.hpp

By restructuring these 3 header files, we can siginficantly reduce the dependency and speed up build time of HotSpot:

$ find hotspot/ -name \*.d -type f | xargs cat | wc
 528052 1054176 32731307
$ find hotspot/ -name \*.d -type f | xargs grep -l jfrEvents.hpp | wc
     65 65 3262

$ time make hotspot:

before:
real 2m19.280s
user 57m15.940s
sys 4m41.332s

after:
real 2m1.446s
user 52m21.708s
sys 3m52.544s