The best solution would be to transform your numeric data in image (or stay with image if you analyse it) and use glsl shader to make calculation and produce images it, or use a software more able to do calculation, as Processing, TouchDesigner or OpenFrameWork and send the result as image via Syphon or Spout to Isadora.
It's difficult to discuss it without knowing from what come your streams, which calculation you want to do and which result you want to obtain…
Be more specific!
When we first introduced the JS actor I wrote a JS script to match the built in 'smoother' actor.
I was able to run hundreds of this actor before any slowdown was apparent.
I believe the JS actor runs in a thread so that it doesn't slow the main Isadora process.
My feeling is that you should be able to do this.
I would be interested in knowing if there is a slight difference between all being processed in 1 JS actor versus splitting the load across multiple actors.
As mentioned above, JS isn't as fast as Isadoras native actors (written in c++).
So you always have the option of building this up as a user actor.