Can you update to the current 10.0.5 and try again?
If it is still slow, then to obtain some more diagnostic information, can you modify the shortcut and add the DebugSQL parameter? For example:
"C:\Program Files\PLSQL Developer\plsqldev.exe" DebugSQL
Reproduce the problem and send me the debug.txt file that is generated in the PL/SQL Developer directory or in the %APPDATA%\PLSQL Developer directory (e.g. C:\Users\\AppData\Roaming\PLSQL Developer).