Use of Linux grep commands

In the past, I used the grep command. I thought it was good and I used it too. However, recently I always thought it was very strange to use it. It was very slow to search for some words in a file. Is my computer configuration really bad?

Grep-r initi_setup_keypad_pai_gpio is unavailable for half a day!

I used the command. Yes, I guess I made a mistake. sate210's partner said he used it like this.

Grep 89c_setup_keypad_cfg_gpio.-R

I tried his method. It's really fast. Now let's take a look at the tips of this grep command.

# Man grep

Look at help

Synopsis
Grep [Options] pattern [file...]
Grep [Options] [-e pattern |-F file] [file...]

-R,-R, -- Recursive
Read all files under each directory, recursively; this is
Equivalent to the-D recurse option.

According to the principle, what I do is correct. What is the problem.