glx-backend broken on Haswell-ULT using the intel xorg driver

Dear maintainer, the cpu-usage of picom v8 using the glx-backend without any options is way too high on an Haswell-ULT using the intel xorg driver. The modesetting-driver, xrender-backend and old compton run fine.

Platform

Debain 10 Buster amd64

GPU, drivers, and screen setup

xserver-xorg-video-intel: 2:2.99.917+git20180925-2

libgl1-mesa-glx: 18.3.6-2

$ inxi -G
Graphics:  Device-1: Intel Haswell-ULT Integrated Graphics driver: i915 v: kernel
           Display: x11 server: X.Org 1.20.4 driver: intel resolution: 1366x768~60Hz
           OpenGL: renderer: Mesa DRI Intel Haswell Mobile v: 4.5 Mesa 18.3.6

$ glxinfo -B
name of display: :0.0
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) Haswell Mobile  (0xa16)
    Version: 18.3.6
    Accelerated: yes
    Video memory: 1536MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.5
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.1
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Haswell Mobile
OpenGL core profile version string: 4.5 (Core Profile) Mesa 18.3.6
OpenGL core profile shading language version string: 4.50
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 3.0 Mesa 18.3.6
OpenGL shading language version string: 1.30
OpenGL context flags: (none)

OpenGL ES profile version string: OpenGL ES 3.1 Mesa 18.3.6
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10

Environment

xfce4

picom version

$ picom --version v8

picom --diagnostics Version: v8

Extensions:

Shape: Yes
XRandR: Yes
Present: Present

Misc:

Use Overlay: Yes* Config file used: None

Drivers (inaccurate):

Intel

Steps of reproduction

execute picom --backend glx using intel's xorg driver.
see a cpu-usage for picom of approx. 35%

Expected behavior

Cpu-usage should not be greater than maybe 2% if idle

Current Behavior

Cpu-usage for picom is approx. 35% on idle.

Other details

picom --backend glx runs fine with the modesetting driver. The xrender-backend runs fine. Compton 0.1~beta2+20150922-1 from Buster's repos does not show this behaviour.

yshui / picom