Something I noticed with 1.5 under Ubuntu 18.04 with a dual-screen system.  

I just purchased so I was googling on one screen under firefox for help, while playing on the other, I noticed that my firefox would become non-responsive.  Exiting flowscape fixed the issue.

I have 16G of memory.  I've attached lshw output if that is helpful.  If I can grab a tty via xterm or SSH while using the app I can run any program to provide any info if so interested.  I also could be running out of swap.  I have only 2G of that and I do see high usage of 16G while running.

lshw:

https://www.dropbox.com/s/wctsa649gz9pbgf/mechonis-hw.txt?dl=0

FlowScape loads a lot of textures so if you dont have a lot of memory, it may slow things down, i would suggest turning texture resolution down if thats the case to see if it makes a difference