Scalable and Flexible Derivation of Custom-Time Bioclimatic and Environmental Summary Variables R package fastbioclim version 0.4.2 Provides an efficient tool for creating custom-time bioclimatic and derived environmental summary variables from user-supplied raster data for user-defined timeframes. The package overcomes computational bottlenecks by automatically switching between an in-memory framework using the 'terra' package to maximize speed for smaller datasets, and an on-disk tiling framework for rasters that exceed available RAM, leveraging 'exactextractr' and 'Rfast' to process data in chunks. The core functions, derive_bioclim() and derive_statistics(), offer a unified interface with flexibility for custom time periods beyond standard quarters and the use of fixed temporal indices, facilitating the creation of temporally-matched environmental variables for ecological and biogeographical modeling. Visit the package website <https://gepinillab.github.io/fastbioclim/> to find tutorials in English and Spanish. Software Depends: R (>= 4.1) Imports: exactextractr, future.apply, glue, progressr, purrr, qs2, Rfast, sf, terra (>= 1.7-0) Suggests: mockery, testthat (>= 3.0.0), withr Gonzalo E. Pinilla-Buitrago <gepinillab@gmail.com> Comprehensive R Archive Network (CRAN) Gonzalo E. Pinilla-Buitrago [aut, cre] (ORCID: <https://orcid.org/0000-0002-0065-945X>), Luis Osorio-Olvera [aut] (ORCID: <https://orcid.org/0000-0003-0701-5398>) GPL (>= 3) 2026-05-08 application/tgz https://CRAN.R-project.org/package=fastbioclim doi:10.32614/CRAN.package.fastbioclim