This is a bit of a WebGL problem, for now. Depending on your browser, if you leave it in a tab, the tab might be put "on-hold", slowing your progression.
On the other hand, I don't want to punish players for going offline and lowering the offline gain. Right now the web release is super convenient for players to quickly try and play the game ; this problem won't happen anymore once the game will be more mature as a desktop app.
Thanks for your feedback!