By default, Replicant devices use the Android software renderer, which relies on the CPU for graphics rendering. Using the dedicated graphics processor (GPU) currently requires non-free software on all of the supported devices. As free-software replacements are not yet available, the GPU cannot be used with Replicant for the moment (see GraphicsResearch). The software rendering is the reason why Replicant devices appear slow at times and is responsible for a lot of crashes that can happen during usage.
llvmpipe has more complete EGL support than the Android software renderer (see #705), so more apps work with it, like Firefox-based browsers or more recent webviews (see #1780). Unfortunately, llvmpipe is still too slow to be the default renderer, but it is possible to switch back and forth between llvmpipe and the Android software renderer.
Make sure ADB is running as root and the system partition is mounted writable.
The following command switches the software renderer implementation:
adb shell "grep -q "ro.libagl=1" /system/build.prop && sed "s/ro.libagl=1/ro.libagl=0/" -i /system/build.prop || sed "s/ro.libagl=0/ro.libagl=1/" -i /system/build.prop"
Then reboot the device. llvmpipe should now be enabled. To switch back to the default Android software renderer, run the above command again and reboot the device.
There are also issues with certain apps when using llvmpipe, although crashes should be less frequent. Usually, the screen stays black if an app does not work with llvmpipe.
In case the screen compositor (SurfaceFlinger) is used with llvmpipe, this could have an effect to other apps. For instance with Gallery and Orbot it happens, that some GUI elements are visible, while the main screen is black. The GUI of Simple File Manager works as long as no image is viewed in fullscreen.