Yutaka Emura

That is probably related to the settings in the Advanced page of the Customize dialog box.
You might want to increase the number in the Minimum File Size to Use Temporary File text box so that your file is under this number if you need to do many replaces. This will avoid using the temporary file, and will load all the file contents to the memory (RAM) so that EmEditor will replace many lines faster.
Please also use the latest version of EmEditor.