How to convert quicly XLSX file to XLS file?

Question

Hello,

I have several XLSX files and I would like to convert each of its files to XLS format in a very fast time, that is to say the conversion of an XLSX file to XLS should take some 100 ms.
To do this, I had thought of using Interop, but it is very slow, at least 8 seconds to convert an XLSX file to XLS.

With Interop, it is very slow to convert XLSX to XLS.

Do you know of a library in C# that allows you to convert an XLSX file to XLS very quickly (ideally 300 ms/file) ?
If yes, could you give me an example of C # code, please?

Thanks.