If "DE error" is high (above 1%) it means that raymarching algorithm is not enough accurate. You will observe some artifacts on the image (noise). When it is lower than 0.5%, the noise is almost invisible on images, especially where there is a lot of details. Always try to play with DE Step factor, because it has the biggest influence on rendering speed. You can use LQ (Low quality) or HQ (high quality) buttons to find "optimal" value of DE factor (it uses DE error value to find optimal DE factor). Then you can tune DE factor manually (check quality of image, not the "DE error").

Try not to change Max. iteration. In most cases It has no influence on rendering speed. Difference is only visible when you are rendering some cross-sections (using limits). Normally "rays" never reach high iteration regions, because may-marching threshold is based on the distance (not on maximum number of iterations). You can watch it in histograms window. On left side there is number of iterations histogram (scale is up to 64 iterations). For mandelbulbs and mandelboxes average number of iterations is between 5 and 20 (peak on histogram is on the left or in the middle).

More details about this topic on:

http://sites.google.com/site/mandelbulber/user-manual#TOC-Ray-tracing-parametersAbout volumetric light it increases "DE error" value, because calculating of light rays uses not accurate ray-marching algorithm (for faster rendering). Fractal details will be still accurate. Don't matter about this. If image looks good, just ignore this. To get faster rendering of volumetric light you can decrease quality of this effect (default is 5) for instance to 1.0. rendering will be 5 times faster but there will be more noise on rays. On high resolution image noise is less visible.

About the formulas, the fastest is "Tglad's folmula Mandelbox". The slowest are Xenodreambuie's formula and all hybrids.

One remark about hybrids. When in hybrid sequence the most of iterations are Mandelbox or some kind of IFS formulas always use "Linear DE Mode". Then ray-marching is much more accurate and you don't have to reduce DE Step factor (rendering is faster). Sometimes you have to try both modes and compare the result.

The slowest effects:

- volumetic light

- not DE shading mode - it uses special algorithm for calculating normal vectors. It's very slow but surface is always smooth.

- ambient occlusion based on rays (not this from Post effects) (speed depends on quality parameter - higher value = higher number of rays = much slower). Sometimes quality = 2 is enough.

- additional light sources. Rendering time = normal time * number of light sources (sometimes faster when lights are not so bright - there is some optimization)