  Magnetohydrodynamical (MHD) dynamos emerge in many different astrophysical
situations where turbulence is present, but the interaction between large-scale
(LSD) and small-scale dynamos (SSD) is not fully understood. We performed a
systematic study of turbulent dynamos driven by isotropic forcing in isothermal
MHD with magnetic Prandtl number of unity, focusing on the exponential growth
stage. Both helical and non-helical forcing was employed to separate the
effects of LSD and SSD in a periodic domain. Reynolds numbers (Rm) up to
$\approx 250$ were examined and multiple resolutions used for convergence
checks. We ran our simulations with the Astaroth code, designed to accelerate
3D stencil computations on graphics processing units (GPUs) and to employ
multiple GPUs with peer-to-peer communication. We observed a speedup of
$\approx 35$ in single-node performance compared to the widely used multi-CPU
MHD solver Pencil Code. We estimated the growth rates both from the averaged
magnetic fields and their power spectra. At low Rm, LSD growth dominates, but
at high Rm SSD appears to dominate in both helically and non-helically forced
cases. Pure SSD growth rates follow a logarithmic scaling as a function of Rm.
Probability density functions of the magnetic field from the growth stage
exhibit SSD behaviour in helically forced cases even at intermediate Rm. We
estimated mean-field turbulence transport coefficients using closures like the
second-order correlation approximation (SOCA). They yield growth rates similar
to the directly measured ones and provide evidence of $\alpha$ quenching. Our
results are consistent with the SSD inhibiting the growth of the LSD at
moderate Rm, while the dynamo growth is enhanced at higher Rm.
