  We present $\mathtt{\text{sl2cfoam-next}}$, a high-performance software
library for computing Lorentzian EPRL spin foam amplitudes. The library
improves on previous codes by many orders of magnitude in single-core
performance, can be parallelized on a large number of CPUs and on the GPU, and
can be used interactively. We describe the techniques used in the code and
provide many usage examples. As first applications, we use
$\mathtt{\text{sl2cfoam-next}}$ to complete the numerical test of the
Lorentzian single-vertex asymptotics and to confirm the presence of the
"flatness problem" of spin foam models in the BF and EPRL cases.
