Building lighting is slow

After migrating a project from version 4.19 to 4.20, build lighting has become really slow.
To be more specific, with version 4.19 lighting calculation went from 0% to 1% after a minute or so, but after moving to version 4.20, the same scene takes 15 minutes or more to go from 0% to 1%.
Speed seems to be ok that, i.e. moving from 1% - 100% seems to take the same time in both versions but I’ve only checked the times on a preview quality level.
Upgrading the whole project to 4.20, or just migrating the level I’m working on from 4.19 to a new empty 4.20 project, has the same result. I haven’t tried building everything back from scratch to check the results,as it would be too time consuming.
Does anyone have any suggestions please?