How to join text values in dataset by both axis 0 and 1 in group?

Question

I have a dataset:

id   f1    f2
1   'a1'  'a2'
1   'b1'  'b2'
1   'c1'  'c2'
1   'd1'  'd2'

I want to create new column text which is join of all text values from f1 and f2 for each id group. desired result is:

id        text
1     'a1. a2. b1. b2'
2     'c1. c2. d1. d2'

How to di that?

I tried this:

df['text'] = ['. '.join(i) for i in zip(df['f1'], df['f2'])]
df['text'] = df.groupby(['id'])['text'].transform(lambda x: '. '.join(x))

But Im looking for best solution for large dataset