See http://netbeans.org/bugzilla/show_bug.cgi?id=187407

Steps to reproduce:
Get project from https://java-nio-charset-enhanced.dev.java.net/source/browse/java-nio-charset-enhanced/branches/JDK-7/j_l_Character_names/?rev=1094

$ svn co -r 1094 https://java-nio-charset-enhanced.dev.java.net/svn/java-nio-charset-enhanced/branches/JDK-7/j_l_Character_names --username guest

Build the project with configuration=parse_UnicodeSpec in NetBeans

$ cd JDK-7/j_l_Character_names
$ /opt/java/jdk1.7.0-b84/fastdebug/bin/java -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=localhost:41614 -Dfile.encoding=UTF-8 -classpath build/classes build.tools.generatecharacter.UnicodeSpec make/tools/UnicodeData/UnicodeData.txt

In a second terminal:
$ jdb -attach 41614
stop at build.tools.generatecharacter.UnicodeSpec:337
resume
 (breakpoint is hit)
next
next
 (it takes a long time to complete)

When a breakpoint is submitted to line 340 and continue is used instead of step over, it completes much faster:

$ jdb -attach 41614
stop at build.tools.generatecharacter.UnicodeSpec:337
stop at build.tools.generatecharacter.UnicodeSpec:340
resume
 (breakpoint is hit)
resume
 (next breakpoint is hit in a short time)

With resume, the application executes much faster than with a step request active. The CPU is consumed by the debuggee and there's no communication with the debugger, therefore there seems to be something inefficient in the debugger backend.