There was a recent update to Mesa to fix boo#1200965.
After the update when I play a video with mplayer the playback is slow.

inxi -aG output:
Graphics:  Device-1: Intel HD Graphics 515 driver: i915 v: kernel bus-ID: 00:02.0 chip-ID: 8086:191e class-ID: 0300
           Display: x11 server: X.Org 1.20.3 compositor: kwin_x11 driver: loaded: modesetting unloaded: fbdev,vesa
           alternate: intel display-ID: :0 screens: 1
           Screen-1: 0 s-res: 3840x2160 s-dpi: 96 s-size: 1016x571mm (40.0x22.5") s-diag: 1165mm (45.9")
           Monitor-1: HDMI-1 res: 3840x2160 hz: 30 dpi: 52 size: 1872x1053mm (73.7x41.5") diag: 2148mm (84.6")
           OpenGL: renderer: Mesa Intel HD Graphics 515 (SKL GT2) v: 4.6 Mesa 21.2.4 direct render: Yes

After the mesa update the last line of the output is:
           OpenGL: renderer: llvmpipe (LLVM 11.0.1 256 bits) v: 4.5 Mesa 21.2.4 compat-v: 3.1 direct render: Yes

For this computer the render has changed from Mesa to llvmpipe which is causing
the slowdown. For the record, on the same computer I have graphic artifacts with the previous version of Mesa. Such artifacts are gone after the update but now the situation is worse. My personal fix is to update to Mesa 21.3.9.