How to handle frame rate drop

In my project I have a lot of writing to files. I am currently working on a simulation and it is needed to extract the mesh in each frame and take screen shots in each frame and write them in file. To be more precise I create a new file for each frame, write the info I want in it and save it (using streamwriter). The simulation itself runs at 70fps. But when I start the writing part which includes the camera and the deformation of the object the frame rate drops. I wanted to know if there is a better way to handle writing to file, instead of doing it in an update method for each frame (I was thinking of threading but Unity is not thread safe).