same glue job running differently when used sample method

I have a csv file with 5 million rows in my s3. I used crawler on the file. I have a custom transformations in my glue job. My issue is, if I use create_dynamic_frame_from_catalog(), it is running very slow, where as if I use create_sample_dynamic_frame_from_catalog() with max sample limit as 5 million, it is working much faster. Why it is happening. i want to speed up the job without using the sample method.