  The Event Horizon Telescope (EHT) has recently released high-resolution
images of accretion flows onto two supermassive black holes. Our physical
understanding of these images depends on accuracy and precision of numerical
models of plasma and radiation around compact objects. The goal of this work is
to speed up radiative-transfer simulations used to create mock images of black
holes for comparison with the EHT observations. A ray-tracing code for general
relativistic and fully polarized radiative transfer through plasma in strong
gravity is ported onto a graphics processing unit (GPU). We describe our GPU
implementation and carry out speedup tests using models of optically thin
advection-dominated accretion flow (ADAF) onto a black hole realised
semi-analytically and in 3D general relativistic magnetohydrodynamics
simulations, low and very high image pixel resolutions, and two different sets
of CPU+GPUs. We show that a GPU with high double precision computing capability
can significantly reduce the image production computational time, with a
speedup factor of up to approximately 1200. The significant speedup
facilitates, e.g., dynamic model fitting to the EHT data, including
polarimetric data. The method extension may enable studies of emission from
plasma with nonthermal particle distribution functions for which accurate
approximate synchrotron emissivities are not available. The significant speedup
reduces the carbon footprint of the generation of the EHT image libraries by at
least an order of magnitude.
