High CPU usage under OpenGL

[MMat]

Pls see also the Edit at the end.

I run Kubuntu 14.04, Xorg version 1.18.3, displaylink driver 1.2.65.

Under Desktop effects -> Advanced, I can choose as Compositing type Xrender and Qt graphycs system Native, in which case everything works just fine except that mouse and windows tear when moved about, and I miss out on all OpenGL candy of course. Or I can choose OpenGL (any version), with Qt Native, where I can choose Tearing Prevention -> Full scene repaints, in which case searing is indeed suppressed, but CPU usage becomes unbearably high: with absolutely nothing running, except for ksysguard,

top - 11:10:52 up 13 min, 12 users,  load average: 0,50, 1,22, 0,99
Tasks: 290 total,   1 running, 288 sleeping,   0 stopped,   1 zombie
%Cpu(s):  8,3 us,  5,2 sy,  0,0 ni, 83,0 id,  3,4 wa,  0,0 hi,  0,1 si,  0,0 st
KiB Mem:   8082416 total,  4234448 used,  3847968 free,    99948 buffers
KiB Swap:  8294396 total,        0 used,  8294396 free.  1810200 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                            
  924 root      20   0 2441780  80636   8100 S  27,2  1,0   7:07.14 DisplayLinkMana                                    
 1855 root      20   0  919540 310784 278716 S   6,6  3,8   1:26.50 Xorg                                               
 3164 myn     20   0  773452  81596  60052 S   4,6  1,0   0:50.00 ksysguard                                          
 2923 myn     20   0 3301140 140852  73208 S   2,7  1,7   0:42.20 kwin                                               
 3058 myn      9 -11  297560  12760   8336 S   1,7  0,2   0:26.66 pulseaudio                                         
 3071 myn     20   0  718124  67348  53032 S   1,0  0,8   0:02.29 konsole                                            
 3441 myn     20   0  868560  42264  34648 S   0,7  0,5   0:05.26 pavucontrol                                        
 3442 myn     20   0  868840  42372  34652 S   0,7  0,5   0:05.70 pavucontrol

while the output below refers to a single instance of vlc running in one of 8 Virtual desktops.

top - 11:14:35 up 17 min, 12 users,  load average: 2,05, 1,35, 1,08
Tasks: 288 total,   1 running, 286 sleeping,   0 stopped,   1 zombie
%Cpu(s): 59,9 us, 19,6 sy,  0,0 ni, 17,2 id,  3,3 wa,  0,0 hi,  0,0 si,  0,0 st
KiB Mem:   8082416 total,  4295256 used,  3787160 free,   104612 buffers
KiB Swap:  8294396 total,        0 used,  8294396 free.  1851656 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                            
  924 root      20   0 2441780  80636   8100 S 112,6  1,0   8:26.92 DisplayLinkMana                                    
 3929 mario     20   0  881904 152076  70344 S  47,7  1,9   0:10.87 chrome                                             
 3878 mario     20   0  924428 184188  81816 S  45,7  2,3   0:11.82 chrome                                             
 4352 mario     20   0 1552428 103208  72456 S  22,9  1,3   0:05.18 vlc                                                
 3586 mario     20   0 1078068 179752  99764 S  18,9  2,2   1:03.46 chrome                                             
 1855 root      20   0  908748 305492 272224 S  13,6  3,8   1:40.50 Xorg                                               
 2923 mario     20   0 3301748 141452  72940 S   8,3  1,8   0:49.17 kwin

Here are my Xorg.0.log file, http://paste.ubuntu.com/23347605/,
my Displaylinkmanager.log file http://paste.ubuntu.com/23347606/
and my FirmwareTrace.log file http://paste.ubuntu.com/23347607/

Also, should this be needed,

$ inxi -Fz
System:    Host: hague Kernel: 4.4.0-42-generic x86_64 (64 bit) Desktop: KDE 4.13.3 Distro: Ubuntu 14.04 trusty
Machine:   System: TOSHIBA product: KIRA BOOK version: PSUC2U-003008
           Mobo: TOSHIBA model: KIRA BOOK version: Version A0 Bios: TOSHIBA version: Version 1.40 date: 01/13/2015
CPU:       Dual core Intel Core i7-5500U CPU (-HT-MCP-) cache: 4096 KB flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) 
           Clock Speeds: 1: 2400.750 MHz 2: 2410.031 MHz 3: 2497.500 MHz 4: 2400.093 MHz
Graphics:  Card: Intel Broadwell-U Integrated Graphics 
           X.Org: 1.18.3 drivers: intel (unloaded: fbdev,vesa) Resolution: 2560x1440@60.0hz, 1920x1080@60.0hz, 1920x1080@60.0hz 
           GLX Renderer: Mesa DRI Intel HD Graphics 5500 (Broadwell GT2) GLX Version: 3.0 Mesa 11.2.0
Audio:     Card-1: Intel Wildcat Point-LP High Definition Audio Controller driver: snd_hda_intel 
           Card-2: Intel Broadwell-U Audio Controller driver: snd_hda_intel 
           Card-3: DisplayLink driver: USB Audio 
           Sound: Advanced Linux Sound Architecture ver: k4.4.0-42-generic
Network:   Card: Intel Wireless 7265 driver: iwlwifi 
           IF: wlan0 state: down mac: <filter>
Drives:    HDD Total Size: 500.1GB (32.7% used) 1: id: /dev/sda model: Crucial_CT500MX2 size: 500.1GB 
Partition: ID: / size: 450G used: 152G (36%) fs: ext4 ID: /boot size: 237M used: 152M (68%) fs: ext2 
           ID: swap-1 size: 8.49GB used: 0.00GB (0%) fs: swap 
RAID:      No RAID devices detected - /proc/mdstat and md_mod kernel raid module present
Sensors:   System Temperatures: cpu: 79.0C mobo: N/A 
           Fan Speeds (in rpm): cpu: N/A 
Info:      Processes: 289 Uptime: 22 min Memory: 2234.5/7893.0MB Client: Shell (bash) inxi: 1.9.17

which shows I use three screens, the 4KUHD of the laptop (a Toshiba Kira) plus two HD external monitors. The DisplayLink device I use is a Dell 3100 USB docking station.

This high CPU usage is a pain both when working (I often use Wolfram Mathematica for computations and graphical display of results and/or numerical simulations) and when having fun (watching a movie, Kodi, the like).

Can anyone help me?

Edit

I have also done the following experiment: since I have two external monitors, I have plugged one of them directly into the HDMI plug in the pc, while keeping the second monitor connected thru the DisplayLink device. Now, the first monitor does not tear, while the second one, i.e. the one connected thru the DisplayLink device, does.

Also, since I have also another pc with Kubuntu 16.04, I have noticed what follows: with both devices connected thru the Dell3100 Docking station, the graphical display cannot be used: certain applications (Konsole) do not display at all, others occupy most of three screens and cannot be resized, tearing is rampant, some applications freeze (others, like xterm work just fine). However, with one monitor connected once again to the pc directly and the other monitor connected to the Dell/DisplayLink dock station, everything works just fine, even though some tearing is still present, but only on the Dell-connected monitor.

All of the above seems to point to some difficulties of your software in handling 2 (or possibly more, I have not tried it) external monitors. Plus tearing, of course, or the flip side of the coin, no tearing with openGL but an abnormal CPU usage.

Am I the only one to have these issues?