Create a folder and put all your text files in it. Then choose Import > MD - Markdown (Directory) and pick that folder you created. It does not matter that they are txt, they will all get imported as separate notes into a notebook with the same name as the folder you created. Each note will use the file name (less the .txt file extension) as the note title.
How long it will take and whether Joplin chokes on super-large imports like this (notes are not often that huge) I do not know.
If they really are that huge it may be worth batching them.