Memory leak/inneficient use of memory when exporting to excel file

Can you paste the specific error.

If you are using a SQL Developer version of 4.0 or higher and exporting using the xlsx format, that memory limitation no longer applies.   See my reply in Issue with copying data from the query result into excel for more information.

In this example i Have limited the memory use... maybe it's not all about memory... but in some point performance goes down for some reason

In case you are using the 4.2 EA1 version, there is this known issue:

Re: Export query data in Excel Format Very Slow in SQL Developer 4.2 early adopter

but I suspect you are just experiencing a slow down related to normal Java VM garbage collection. In that case, I recommend not limiting the -Xmx memory setting unless you do not have sufficient physical memory on the machine.  If you want to try different garbage collection algorithms, read through Java HotSpot VM Options in the JDK 8 link for Windows. Maybe something like -XX:+UseG1GC can help -- it seems to be the only algorithm that gets Task Manager to show an actual reduction in memory use when closing connections, for example.

Possibly, but SQL Developer relies on Apache POI so it is not all under our control. 

Keep in mind that Task Manager does not show the exact amount of memory in use, just some larger amount allocated in anticipation of use.  Use a memory profiler if you want to track actual use.  If the absolute amount of memory allocated within the -Xmx limit bothers you, considerable relief is possible by using a 32-bit JDK rather than 64-bit.  I personally stay with 64-bit installs after moving to a 64-bit machine with 8 GB of RAM over 4 years ago.