ENH: Warning with only one column used in `apply` causing performance issues
#### Is your feature request related to a problem?

From what I've seen in python classes: Handling a huge data frame, especially beginners using `apply` on huge data frames don't know that it loads the whole frame while looping.

[Example](https://stackoverflow.com/a/62968313):
Consider following function and data frame
```python
  import pandas as pd

  def complex_function(x, y=0):
      if x > 5 and x > y:
          return 1
      else:
          return 2

  df = pd.DataFrame(data={'col1': [1, 4, 6, 2, 7], 'col2': [6, 7, 1, 2, 8]})
```
For a greater data frame, consider performance differences with
```python
df['col1'] = df['col1'].apply(complex_function)
```
and the much less efficient
```
df['col1'] = df.apply(function(x) {complex_function(x['col1'])})
```
#### Describe the solution you'd like

Just a small warning that, if only one column is accessed within apply body. Something like:
```
Warning: Only one column seems to be accessed, causing performance issues
```
#### Describe alternatives you've considered

We just leave it as it is, no great differences for most users
