I have not implemented IDE, debugger or JVM, so I can not be sure that the things are going exactly as I will explain here.
But. When code is running with debugger the JVM interprets the code until it meets breakpoint. Then it stops and calls debugger (IDE).
JVM’s do not support conditional breakpoints, so IDE’s use a “hack” to accomplish this feature. The IDE simply adds a normal breakpoint. Every time a breakpoint is hit, the IDE evaluates the expression itself before alerting the user, if the evaluation is false, it sends the “continue” command.
Now examine your examples. In second example JVM performs such call only once. In first example this is done 100000 times. Each time JVM calls debugger and waits until it interprets the condition and sends to JVM command “continue” (exactly as you can do manually when you are debugging your code). Obviously 100000>1, so this process takes time.
EDIT: the next 2 paragraphs were written as an not proven assumption only. The experiments of OP showed that they are wrong. I however do not want to remove them completely: let’s interpret this as a theoretical thinking and improvement proposal for Eclipse team.
Concerning IntelliJ vs Eclipse. Again, this is assumption only. I saw that IntelliJ works much slower with conditional breakpoints. I know also that conditional breakpoints in IntelliJ do not support some elements of java programming language (e.g. anonymous inner classes). I can conclude that IntelliJ probably compiles the code that you write as a condition of your brekepoint using language other than java (for example groovy or something). This probably causes additional performance degradation.
I also know that eclipse does not use standard javac compiler but its own compiler that has a lot of cool features. I can assume that probably conditional breakpoints in eclipase are compiled as a part of your code, i.e. actually compiler automatically creates code like your example number 2. If this is correct such code will run almost as quickly as code that contains manually written if statement.