Severe performance regression was observed on X Server 1.18 with older graphics cards

[Kevin Brace]

Hi,

I am hoping someone can track down a bad commit that led to severe performance regression likely caused by X Server 1.18.
The other day, I was trying to figure out why a computer with Xubuntu 16.04.2 was running so slowly.
The thing is, as long as I used Lubuntu 12.04 or Xubuntu 14.04, I was getting the performance I was expecting (fairly fast).
Here is the spec of the computer.

CPU: AMD Opteron 165 (1.8 GHz, 1 MB L2 cache, dual core) 
Mainboard: ASRock 939Dual-SATA2
RAM: 2.25 GB
Graphics: SiS 305 (AGP, 32 MB)

First, I noticed severe performance regression on Xubuntu 16.04.2.
Xubuntu 16.04.2 uses Linux 4.10 and X Server 1.18.
Xubuntu 14.04 (stock) uses Linux 3.13 and X Server 1.15.
Lubuntu 12.04 (stock) uses Linux 3.2 and X Server 1.11.
Please note that all version used xf86-video-sis (it is renamed as xserver-xorg-video-sis for Debian / Ubuntu), and they all used the version from the upstream freedesktop.org repository.
For Xubuntu 16.04, Canonical no longer builds xserver-xorg-video-sis, so I compiled the upstream code and installed it.
In other words, I am not running on xf86-video-vesa.
    Since I noticed this severe performance regression, I wanted to do one experiment.
I upgraded Xubuntu 14.04 to 14.04.5.
By doing this, it will replace Linux 3.13 with 4.4 and Xserver 1.15 with 1.18.
I basically got identical results to Xubuntu 16.04.2.
Now, both Xubuntu 14.04.5 and 16.04.2 both are identical in terms of performance when I use xf86-video-sis.
I suspect, something happened between X Server 1.15 and 1.18 to trigger this severe performance regression.
    When I say, "severe performance regression," I mean that the performance of the computer feels like I am running Intel Pentium III at 500 MHz in terms of 2D graphics performance.
The computer is now so painful (psychologically stressful) to use. 
Furthermore, it appears that xf86-video-vesa is faster than the supposedly accelerated xf86-video-sis, but due to this particular graphics card's issue, I cannot use xf86-video-vesa since I will be restricted to 640 x 480 screen resolution.
I hope this serious performance bug can be fixed by the time of X Server 1.20 release and maybe backported to 1.18 and 1.19.

Regards,

Kevin Brace