On Kubuntu 16.04. I have about 15 tabs open in the first window. With a second window open and minimized, with about 5 tabs open, the first window becomes sluggish. Switching between tabs can take several seconds. After closing the second window, response is back to normal.
total used free shared buff/cache available
Mem: 20550988 10877480 618472 1689172 9055036 7516452
Swap: 0 0 0