You really cannot compare performance to other fixed-path VSTi's directly.
They are easier to optimize, simply because they are fixed.
That said, I believe there still is stuff that can be done to speed up the synth.... and I promise to take a look at it
If you have a specific setup that you think takes excessive amounts of CPU, please do send me a patch so I can take a look and perhaps remove or optimize the bottleneck.
Please note however that the power you are given with a modular synth has a price. This power is freedom to let you patch modules together in endless ways, but that freedom also allows you create setups that will bring even a modern PC to its knees. If you for instance route an OSC into a reverb module then into a 16xunison module and finally into a 8x polyphony notepitch module, that is like asking for trouble, but that setup is of course also far from optimal.
Some general patch performance tips are:
* Try using as few modules as possible to get the desired result.
Remember, every module will eat a tiny chunk of your CPU.
* If you use polyphony, place all modules that don't need polyphony after the notepitch module.
Typically this means effect modules. If you place a reverb _before_ an 8x polyphonic notepitch module the synth will worstcase internally run 8 instances of the reverb. Expensive stuff, and you won't really hear the difference.
* Place any unison module as close to the osc as possible in the signal chain.
* Only use stereo where it counts.
* Be aware that some modules are more expensive to use than others.
Simplified, Osc's, simple filters and Modulators are fast, while Effects, EQ filters and unison/polyphony is slow.
The code in KarmaFX Synth is highly optimized, in many cases by writing inner loops by hand in FPU/SSE assembler. While this is all fine, things could be made even faster by cutting corners at the cost of a slight quality loss. For instance, modulation doesn't really have to be updated every sample, but could be updated at a lower frequency, like say every fourth sample, in many cases without you would hear a difference.
However, KarmaFX Synth has never walked down that path. I truly believe that quality matters and is far more important than speed. I don't think anyone would choose to use a software synthesizer, if they can get better quality out of their old hardware synthesizers. If you think differently, let me know.