You do not state which release of SQL Developer you use. A complete answer depends on that, but standard advice for xls formats is...
1. Export to csv format instead.
2. Increase the Java VM heap size (via the -Xmx parameter).
A old discussion that covers this is Re: Export to Excel Hangs
Starting with the 4.0.x releases, the -Xmx parameter may be found in a product.conf file under your user settings folder. For prior releases, it lives in sqldeveloper.conf under your SQL Developer install folder.
The reason for the slow down is the API used keeps all the data in memory. Recent releases of SQL Developer use a newer API for the xlsx format that does not have this problem.
SQL Developer Team
Data fetched in the SQL editor set fast but when I am saving in excel sheet is taking very very long time and it it is not acceptable.
You may think that 'Data fetched in the SQL editor set fast' but you can only see a SMALL SET of data.
The issue is directly related to the amount of data you are retrieving and saving. That amount depends on the average size of each row and the number of rows.
The amount of data is then expanded SIGNIFICANTLY when you use XML format because the begin/end tags take a significant amount of space.
Excel can read delimited data so you can always reduce the amount of data by saving to a delimited file instead of XML. If, as Gary suspects you are using an older version of Sql Developer then you should download and use the latest released version instead.