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1.13.0 slow down in a repeated loop on list column #4658

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@sandoronodi

I have noticed a huge performance drop in data.table loop operations, possibly due to the new version upgrade

library(data.table)
library(microbenchmark)

dt <- data.table('id'=1:20000,
                 'list_col'=sample(c('', '', 'a', 'a:b', 'a:b:c'), 20000, TRUE))
feature <- 'list_col'

microbenchmark(
  long_dt <- dt[, c("id", feature), with = FALSE][
    , feature_names := {
      x <- get(feature)
      stringr::str_split(x, ':')
    }][
      , .(
        feature_names = paste0(feature, "_", unlist(feature_names))
      )
      , by = "id"]
  , times = 10
  , unit = 'ms'
)

data.table 1.12.8, default settings, using 6 threads:

      min       lq     mean   median       uq      max neval
 122.2447 149.6991 173.3268 183.5777 193.9876 201.7234    10

data.table 1.13.0, default settings, using 6 threads:

      min      lq     mean   median      uq      max neval
 12820.75 12913.1 12989.59 13007.94 13065.1 13097.85    10

Also, I have tried several different threads and throttle combinations, but have seen no improvements at all

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