Icarus, Electra, Nemesis, Saurus and FireBird do have real multicore CPU support which is a FEATURE, not a bug or bad software design. If 'Allow threaded processing' is disabled the plugins can only use a single CPU core for processing and this can have negative effects on the audio process (crackes).
Plugins which do not come with multicore CPU support will of corse run in the same way when the feature is disabled, since they always only run on a single core.
All modern hosts have multicore CPU support enabled by default, since it is a mayor performance advantage.
The latest versions of our plugins do not need the setting 'use fixed size buffers', since we did a workaround especially for FL studio. However it might help to reduce CPU spikes or crackles when very aggressive timings are choosen with the ASIO soundcard latency.
FL Studio is the only host which forwards random (!) buffer sizes to the VST plugin. It processes buffer sizes with extreme jitter like 256, 2, 59, 29, 19, 127, 1, 64 ... The result are CPU spikes which can cause clicks when very low latencies are choosen. Every other hosts requests equal sizes like 128,128,128... We had contacted image line several times during the last 10 years, but the problem was continously ignored. Every VST developer can proof the bad behaviour by logging the sampleFrames number which is submitted with processReplacing (float **inputs, float **outputs, VstInt32 sampleFrames) .
I have measured various hosts and FL Studio no other one behaves like this.
Best Regards,
Markus