Inserting "bigger" data into PostgreSQL makes the system faster?
I can't reproduce this without more detail about your setup and your programs, so the following is just an educated guess. It's conceivable that your observation is due to TOAST. Once a text field exceeds a certain size, it is stored in a physically separate table. Therefore, the main table is actually smaller than in the case where all the text values are stored inline, and so searches could be faster.