nice to welcome you here to our forum and thanks for your observation.
Does your observation regarding the performance only apply to the PDF export of List & Label or can you also reproduce the same behavior in the context of a preview export? To make it export to a List & Label preview file instead of a PDF file, you can use this code here:
ExportConfiguration expConf = new ExportConfiguration(LlExportTarget.Preview, @"myPreviewFile.ll", @"myProjectFile.lst");
The reason why I ask is, that during the PDF export, the handling of the fonts can lead to performance issues, if the font used in the project file does not support the characters to be displayed, for example, and an alternative font has to be selected during the PDF export, which has to be searched for in the system. This would fit very well with the fact that the reports for the Asian variants - here Japanese and Chinese - take considerably longer than the report with the English data/texts. Because here every single character has to be searched out and replaced in the system, so that it can be embedded as a font in the PDF document. If, however, a suitable font for Japanese or Chinese texts is already used in the project, the time-consuming selection of the font for the PDF export is no longer necessary and it should be much faster. Whether in your case the font “MS Microsoft Unicode” is sufficient, I can not say yet, because it also depends on the correct display of the respective characters - but it would be worth a try.
Finally, it is also interesting to know with which version (incl. Service Pack) of List & Label you are working.
I hope that the information provided can already help a little to narrow down the cause - or even solve it.