In the thin film area, it is well-known that deposition rate impacts the morphology but tools to quantify online fast growth processes are scarce. Here we show that surface differential reflectivity spectroscopy (SDRS) can be used in real time to follow silver nanoparticle growth during sputtering deposition. The main experimental challenge was to avoid noise and saturation due to the plasma emission and to obtain a reasonable signal/noise ratio to monitor fast deposition. A specific setup was designed resulting in an acquisition speed in the range of hundreds of milliseconds and used to investigate the growth of silver on alumina as a test example. The evolution of the size, density and aspect ratio of growing silver islands were determined by modelling their plasmonic response and compared with previous results obtained at a much lower growth rate using physical vapour deposition (Lazzari and Jupille 2012 Nanotechnology 23 135707). During room-temperature sputter deposition, coalescence leads to significantly larger and flatter aggregates compared to evaporation at the same coverage. However, both deposition techniques lead to similar nucleation and growth behaviours. Higher substrate temperature (575 K) did not change the trend and a sticking coefficient close to one was found. The observed evolution of the particle aspect ratio is discussed in terms of supersaturation and flux driven hindrance of the coalescence.