I/O operations in nifti.io are optimized to be very fast and require minimal RAM. GZipped NII files are not supported (due to inconsistent indexing into files). Specification of the NIfTI-1.1 format can be found here LINK.
These methods were developed to support freedom in statistical modelling that currently does not exist in standard neuroimage processing suites. If a statistical model can be deployed in R, then nifti.io can: 1) bring 3D and 4D neuroimaging data into R to be modelled with parallelization and efficient memory use in mind 2) and put statistical results back into brain-space for visualization, interpretation, and further analysis.
Citation: Koscik TR, Man V, Jahn A, Lee CH, Cunningham WA. Decomposing the neural pathways in a simple, value-based choice. Neuroimage. 2020;214: 116764. LINK