Due to recent high-resolution ALMA observations, there is an accumulating evidence for presence of giant planets with masses from ~0.01M_J_ to a few M_J_ with separations up to 100au in the annular structures observed in young protoplanetary discs. We point out that these observations set unique 'live' constraints on the process of gas accretion on to sub-Jovian planets that were not previously available. Accordingly, we use a population synthesis approach in a new way: we build time-resolved models and compare the properties of the synthetic planets with the ALMA data at the same age. Applying the widely used gas accretion formulae leads to a deficit of sub-Jovian planets and an overabundance of a few Jupiter mass planets compared to observations. We find that gas accretion rate on to planets needs to be suppressed by about an order of magnitude to match the observed planet mass function. This slower gas giant growth predicts that the planet mass should correlate positively with the age of the protoplanetary disc, albeit with a large scatter. This effect is not clearly present in the ALMA data but may be confirmed in the near future with more observations.