Found the solution, importing too much topics in a single category hangs the nodebb, import some data than move it to some temporary category. Thasn import remaining data, meanwhile delete mongodb import Garbaze in using these mongo commands
But keep in mind this will also delete the history of import will result in duplicate categories get imported or the posts, to prevent this keep writing post ids and category ids which you imported and delete them from the old database.
And thank you so much for pointing me to the right direction. I was not clear how to delete/update these database settings. I have used Mongo_Express for this and cleaned the database according to your referenced topic. And it finally cleared the unwanted topics from my forum.