Ubuntu – Choppy videos and animations since 17.10

After a fresh Ubuntu 17.10 installation all animations (like this or this one in the background) and videos (f.i. in VLC or on YouTube, like this one) do have a small stutter every second.

Interestingly, YouTube full-screen videos play smoothly, even in 1080p. If another window gets into the foreground (by Alt+Tab), when watching a video in full-screen, then the video begins to stutter again. Per second there is on short stutter.

When the same 1080p video is minimized into a normal browser window, then it lags again.

This is true for Firefox and the Opera browser.

I've never experienced something similar on previous Ubuntu releases with the same hardware.

I'm running Ubuntu on Xorg, not Wayland, because of mouse lags under Wayland.

How can I get rid of those lags and stutters respectively?

That's my hardware:

$ lspci -nnk | grep -i VGA -A2
00:02.0 VGA compatible controller [0300]: Intel Corporation 3rd Gen Core processor Graphics Controller [8086:0166] (rev 09)
    Subsystem: Toshiba America Info Systems 3rd Gen Core processor Graphics Controller [1179:fb31]
    Kernel driver in use: i915

I'm really desperate and appreciate every help. Thanks in advance!