FMOD 2.0 and resonance audio soundfield

I have a question regarding FMOD 2 and resonance audio (and Unity).
I have some issues with the ambisonic soundfields in FMOD. When I run an ambisonic soundfile through the soundfield the performance increases dramatically to 90% - 100%. On a faster computer I could keep it under 90% for some time, but on my MacBook Pro it went into 100% immediately when playing a soundfield ambisonic file.
Do any of you have experience using soundfields and ambisonic files in FMOD 2.0 using the resonance audio spatializer?
I have both tried using pure 1st order ambix sounds as well as pre-baked 1st order ambisonic wave files, and it all pushes the performance to more than 90% using one sound. When I look at the profiler, the sound from the resonance audio spatializer is almost the only thing taking up the performance. I managed to play up to almost 150 resonance audio sound sources simultaneously with OK performance (still something like 50-60% of the CPU though), but as soon as just one single soundfield with an ambisonics file was played the performance went straight to 100%.
Are there settings I should know of in either Unity or FMOD or is this a known issue?

I really hope you can help me with this.

Im using FMOD 2.00.05 and Unity (fairly new 2019 version) by the way.
And the Unity project does almost not have anything else then boxes and planes.