Thanks for the detailed feedback, I will investigate the performance problems. The audio stutters have been introduced in the latest update I think, I might just revert those changes to the sound bus.
I am also on Mint + Firefox and don't notice the same behaviour, though it is a newer laptop. You could try the linux native client of the game, it doesn't have the audio stutters and performance should be better.
The "days per in-game tick" increase when switching to a higher level map - the idea is that you adjust your frame rate to not get bored while your ships need much longer time to travel to other star systems instead of planets.