I have a script that takes only 4 seconds to process in V 1.12.8, and more than 24+ hours when using V 1.13.0. All other variables are the same (computer, r and RStudio versions, dataset, etc.)

All relevant and detailed information about this issue is contained in Stack Overflow.

https://stackoverflow.com/questions/63105711/why-data-table-unnesting-time-grows-with-the-square-of-number-of-rows-for-a-spec
I am not a programmer, so I am not able to go through the code. Below you will find the sessionInfo() reg\rding thje environment AFTER changing back to V 1.12.8.

Please let me know whether I can be of any further help.

BTW, congratulations for this amazing gamecahnger package.

Regards,

Fabio.

sessionInfo()
R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18363)

Matrix products: default

locale:
[1] LC_COLLATE=Portuguese_Brazil.1252 LC_CTYPE=Portuguese_Brazil.1252 LC_MONETARY=Portuguese_Brazil.1252
[4] LC_NUMERIC=C LC_TIME=Portuguese_Brazil.1252

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] microbenchmark_1.4-7 data.table_1.12.8 lubridate_1.7.9 stringi_1.4.6 runner_0.3.7 e1071_1.7-3
[7] ggplot2_3.3.2 stringr_1.4.0 magrittr_1.5

loaded via a namespace (and not attached):
[1] Rcpp_1.0.5 pillar_1.4.6 compiler_4.0.2 class_7.3-17 tools_4.0.2 digest_0.6.25 packrat_0.5.0 evaluate_0.14
[9] lifecycle_0.2.0 tibble_3.0.3 gtable_0.3.0 pkgconfig_2.0.3 rlang_0.4.7 rstudioapi_0.11 yaml_2.2.1 xfun_0.16
[17] withr_2.2.0 dplyr_1.0.0 knitr_1.29 generics_0.0.2 vctrs_0.3.2 grid_4.0.2 tidyselect_1.1.0 glue_1.4.1
[25] R6_2.4.1 rmarkdown_2.3 purrr_0.3.4 scales_1.1.1 ellipsis_0.3.1 htmltools_0.5.0 colorspace_1.4-1 tinytex_0.25
[33] munsell_0.5.0 crayon_1.3.4