Parse JSON efficiently

Say I have the below table

library(jsonify)
library(data.table)

df <- data.table(col = c('{"geo":"USA","class":"A","score":"99"}'
                         , '{"geo":"Hawaii","class":"B","score":"83"}'
                         )
                 ); df
col
                                      <char>
1:    {"geo":"USA","class":"A","score":"99"}
2: {"geo":"Hawaii","class":"B","score":"83"}

and I wanted to extract certain fields into columns

# fields to extract
x <- c('geo', 'class')

# extract
df[, (x) := {y = lapply(col, \(j) from_json(j)) |>
                    lapply(`[`, x)
             a = sapply(y, `[`, x[1])
             b = sapply(y, `[`, x[2])
             .(a, b)
             }
   ]

Above works but 2 problems:

1. Manual. If I had, say, 40 fields to extract I would have to write out many lines of code
2. Not memory efficient nor fast

Real data set has millions of rows and each row has about 50 fields. Currently takes about 15 mins to parse 2 fields for 1e5 rows. Further more, even after completion, my PC is lethargic. Ctrl + Alt + Del confirms a lot of memory is taken up. gc() does not solve either.

Below are code to create larger sample data

# Function to generate sample data
generate_sample_data <- \(n_rows = 10)
{
  # Generate random data for each field
  geo_options <- c("USA", "Hawaii", "Canada", "Mexico", "UK")
  class_options <- c("A", "B", "C", "D")
  
  # Create the data table
  df <- data.table(col = replicate(n_rows
                                   , paste0('{"geo":"', sample(geo_options, 1), '",'
                                            , '"class":"', sample(class_options, 1), '",'
                                            , '"score":"', sample(60:100, 1), '",'
                                            , '"extra_field":"', sample(LETTERS, 1), '",'
                                            , '"timestamp":"', as.character(Sys.time() + sample(0:100000, 1)), '"}'
                                            )
                                   )
                   )
  
  return(df)
}

# Generate a data table with 15 rows
df <- generate_sample_data(1e6); df[1:3]

try using fromJSON function

jsonlite::fromJSON(sprintf("[%s]", toString(df[[1]])))
      geo class score extra_field                  timestamp
1  Canada     C    63           R  2025-01-21 02:56:42.58203
2      UK     A    71           Z 2025-01-21 12:15:20.582483
3  Hawaii     D    79           K 2025-01-21 14:43:41.582883
4  Canada     C    68           M 2025-01-21 01:40:20.583218
5  Mexico     B    92           X 2025-01-21 05:26:35.583423
6  Hawaii     D    71           R 2025-01-21 05:09:51.583672
7  Canada     D    95           F 2025-01-21 10:36:04.583794
8      UK     B    84           O 2025-01-21 13:11:29.583986
9  Mexico     A    92           W  2025-01-21 15:21:13.58412
10     UK     A    75           Z 2025-01-21 12:47:03.584297
11 Hawaii     B    70           T 2025-01-20 13:31:34.584423
12     UK     A    88           Q 2025-01-21 09:46:32.584641
13 Mexico     B    64           K 2025-01-21 11:03:20.584838
14 Hawaii     D    63           J 2025-01-21 13:53:28.585002
15     UK     D    83           B 2025-01-21 04:03:55.585202