ds.map(f, num_proc=10) is slower than df.apply

### Describe the bug

pandas columns: song_id, song_name
ds = Dataset.from_pandas(df)

def has_cover(song_name):
if song_name is None or pd.isna(song_name):
return False
return 'cover' in song_name.lower()

df['has_cover'] = df.song_name.progress_apply(has_cover)
ds = ds.map(lambda x: {'has_cover': has_cover(x['song_name'])}, num_proc=10)

time cost:
1. df.apply: 100%|██████████| 12500592/12500592 [00:13<00:00, 959825.47it/s]
2. ds.map: Map (num_proc=10):  31%
 3899028/12500592 [00:28<00:38, 222532.89 examples/s]

### Steps to reproduce the bug

pandas columns: song_id, song_name
ds = Dataset.from_pandas(df)

def has_cover(song_name):
if song_name is None or pd.isna(song_name):
return False
return 'cover' in song_name.lower()

df['has_cover'] = df.song_name.progress_apply(has_cover)
ds = ds.map(lambda x: {'has_cover': has_cover(x['song_name'])}, num_proc=10)

### Expected behavior

ds.map is ~num_proc faster than df.apply

### Environment info

pandas: 2.2.2
datasets: 2.19.1