Here are some fast and dirty benchmarks of various gtk+ engines installed on my system, using app-benchmarks/gtkperf-0.40.
Notes on the hardware:
CPU: Athlon 64 X2 4600+
Graphics: nVidia 7600GT, DVI 1440x900 @ 60Hz
RAM: 4GB DDR2-667
Mobo: ASUS M3N78-VM
Notes on the testing environment:
OS: Gentoo Linux (duh)
Kernel: Linux 2.6.27-gentoo-r2 #3 SMP PREEMPT x86_64
nvidia-drivers: 177.82
CFLAGS: -march=athlon64 -O2 -msse3 -fomit-frame-pointer
DE: Xfce 4.4.3
- Xfwm4 with Composite enabled, effects: drop shadows & transparency
- Open applications: 1 instance each of www-client/mozilla-firefox, app-editors/gvim, xfce-extra/terminal
- Cairo: 1.6.4, compiled with glitz support. Not all engines use Cairo, but those that do should benefit from a small speed increase.
The gtk+ engines are all available in Portage. If you're not on Gentoo, look in your distribution's repositories or check here. Custom themes are noted with *. These are personal themes I've made, nothing more than simple color modifications of an existing freely available theme. No additional images are used, so rendering time should not be affected.
All tests were conducted 3 times, using a Test Round setting of 100. I picked the best score of the 3, as I was looking for best-case usage conditions. The results are ranked in order from fastest to slowest.
Engine: Mist
Theme: Mist
GtkEntry - time: 0.01
GtkComboBox - time: 0.53
GtkComboBoxEntry - time: 0.47
GtkSpinButton - time: 0.09
GtkProgressBar - time: 0.03
GtkToggleButton - time: 0.13
GtkCheckButton - time: 0.06
GtkRadioButton - time: 0.24
GtkTextView - Add text - time: 0.28
GtkTextView - Scroll - time: 0.13
GtkDrawingArea - Lines - time: 0.28
GtkDrawingArea - Circles - time: 0.35
GtkDrawingArea - Text - time: 0.29
GtkDrawingArea - Pixbufs - time: 0.16
---
Total time: 3.07
Engine: Xfce
Theme: Xfce
GtkEntry - time: 0.03
GtkComboBox - time: 1.12
GtkComboBoxEntry - time: 0.50
GtkSpinButton - time: 0.06
GtkProgressBar - time: 0.04
GtkToggleButton - time: 0.14
GtkCheckButton - time: 0.07
GtkRadioButton - time: 0.06
GtkTextView - Add text - time: 0.27
GtkTextView - Scroll - time: 0.17
GtkDrawingArea - Lines - time: 0.27
GtkDrawingArea - Circles - time: 0.35
GtkDrawingArea - Text - time: 0.31
GtkDrawingArea - Pixbufs - time: 0.16
---
Total time: 3.56
Engine: Rezlooks
Theme: Blue Ink*
GtkEntry - time: 0.07
GtkComboBox - time: 0.95
GtkComboBoxEntry - time: 0.65
GtkSpinButton - time: 0.06
GtkProgressBar - time: 0.03
GtkToggleButton - time: 0.28
GtkCheckButton - time: 0.28
GtkRadioButton - time: 0.39
GtkTextView - Add text - time: 0.34
GtkTextView - Scroll - time: 0.15
GtkDrawingArea - Lines - time: 0.27
GtkDrawingArea - Circles - time: 0.36
GtkDrawingArea - Text - time: 0.29
GtkDrawingArea - Pixbufs - time: 0.17
---
Total time: 4.31
Engine: Industrial
Theme: Industrial
GtkEntry - time: 0.08
GtkComboBox - time: 1.52
GtkComboBoxEntry - time: 1.04
GtkSpinButton - time: 0.12
GtkProgressBar - time: 0.05
GtkToggleButton - time: 0.59
GtkCheckButton - time: 0.35
GtkRadioButton - time: 0.39
GtkTextView - Add text - time: 0.39
GtkTextView - Scroll - time: 0.21
GtkDrawingArea - Lines - time: 0.28
GtkDrawingArea - Circles - time: 0.36
GtkDrawingArea - Text - time: 0.28
GtkDrawingArea - Pixbufs - time: 0.17
---
Total time: 5.86
Engine: Glider
Theme: Glider
GtkEntry - time: 0.04
GtkComboBox - time: 1.93
GtkComboBoxEntry - time: 1.62
GtkSpinButton - time: 0.42
GtkProgressBar - time: 0.02
GtkToggleButton - time: 0.25
GtkCheckButton - time: 0.19
GtkRadioButton - time: 0.32
GtkTextView - Add text - time: 0.37
GtkTextView - Scroll - time: 0.29
GtkDrawingArea - Lines - time: 0.28
GtkDrawingArea - Circles - time: 0.36
GtkDrawingArea - Text - time: 0.31
GtkDrawingArea - Pixbufs - time: 0.17
---
Total time: 6.59
Engine: Pixmap
Theme: Elegant Autumn*
GtkEntry - time: 0.09
GtkComboBox - time: 1.64
GtkComboBoxEntry - time: 1.34
GtkSpinButton - time: 0.24
GtkProgressBar - time: 0.17
GtkToggleButton - time: 0.52
GtkCheckButton - time: 0.48
GtkRadioButton - time: 0.89
GtkTextView - Add text - time: 0.69
GtkTextView - Scroll - time: 0.22
GtkDrawingArea - Lines - time: 0.26
GtkDrawingArea - Circles - time: 0.36
GtkDrawingArea - Text - time: 0.28
GtkDrawingArea - Pixbufs - time: 0.17
---
Total time: 7.37
Engine: Clearlooks
Theme: Glossy
GtkEntry - time: 0.08
GtkComboBox - time: 1.93
GtkComboBoxEntry - time: 1.45
GtkSpinButton - time: 0.40
GtkProgressBar - time: 0.29
GtkToggleButton - time: 0.61
GtkCheckButton - time: 0.50
GtkRadioButton - time: 0.59
GtkTextView - Add text - time: 0.41
GtkTextView - Scroll - time: 0.32
GtkDrawingArea - Lines - time: 0.27
GtkDrawingArea - Circles - time: 0.35
GtkDrawingArea - Text - time: 0.31
GtkDrawingArea - Pixbufs - time: 0.18
---
Total time: 7.68
Engine: Candido
Theme: Graphite Light
GtkEntry - time: 0.08
GtkComboBox - time: 2.10
GtkComboBoxEntry - time: 1.86
GtkSpinButton - time: 0.17
GtkProgressBar - time: 0.26
GtkToggleButton - time: 0.63
GtkCheckButton - time: 0.53
GtkRadioButton - time: 0.60
GtkTextView - Add text - time: 0.48
GtkTextView - Scroll - time: 0.25
GtkDrawingArea - Lines - time: 0.28
GtkDrawingArea - Circles - time: 0.36
GtkDrawingArea - Text - time: 0.29
GtkDrawingArea - Pixbufs - time: 0.17
---
Total time: 8.05
Engine: Aurora
Theme: Aurora
GtkEntry - time: 0.47
GtkComboBox - time: 3.78
GtkComboBoxEntry - time: 3.50
GtkSpinButton - time: 0.96
GtkProgressBar - time: 0.31
GtkToggleButton - time: 1.53
GtkCheckButton - time: 1.29
GtkRadioButton - time: 1.66
GtkTextView - Add text - time: 0.58
GtkTextView - Scroll - time: 0.46
GtkDrawingArea - Lines - time: 0.31
GtkDrawingArea - Circles - time: 0.38
GtkDrawingArea - Text - time: 0.32
GtkDrawingArea - Pixbufs - time: 0.19
---
Total time: 15.73
As you can see, the older engines are generally the fastest, with the more modern Rezlooks engine coming in close behind. Though they're generally not as attractive, the old Mist and Xfce engines turn in very respectable rendering times. The Pixmap engine actually doesn't score too well, coming in at the lower middle of the pack. This is despite many reports I found via Google that suggest it's one of the best-performing engines out there. Not so much; it's about average.
But by far the worst performing engine is Aurora. Now, to be fair, Aurora does many graphical tricks the other engines do not. It came along some time after old engines like Pixmap, Industrial, Mist, and Glider. It features animated scrollbars, gauges, and many possible styles of dropdowns and arrows. In short, it's fully loaded. Yet it also doesn't seem to be optimized; at 15.73 seconds, it's almost twice as slow as the nearest contender, Candido.
The results for the Aurora engine were so dismal that I re-ran gtkperf another 3 rounds, thinking something was amiss. Every result turned in times between 15 and 16 seconds. Clearly, Aurora isn't the engine to use if you're on old hardware.
Conclusion:
Remember, these are down and dirty benchmarks. Cherry-picking the best time out of 3 runs may not be the most fair way of measurement, but since no single result varied more than 2 seconds either way, it can be considered pretty well representative of the engine's overall capabilities.
If you're on less capable hardware, the Mist and Xfce engines will go far. If you want something prettier, stick with Rezlooks. I have several screenshots of Rezlooks-based environments in my devspace. It's quite flexible, and it's still in the top three fastest engines, despite including goodies like subtly animated progress bars and gauges.
But even on my fairly powerful workstation, newer engines like Candido and Aurora were noticeably slower, suggesting they might not be a good fit for older hardware. Clearlooks and Pixmap are middle-of-the-road choices; neither has much of an advantage. It comes down to which engine you think has prettier themes.
Me? I stick with Rezlooks. And occasionally Clearlooks (the Glossy theme makes for a good wintry desktop foundation), and very occasionally I'll find a decent Pixmap theme that's worth modding for my system. Otherwise, it's Rezlooks all the way.