Yes I see I was wrong - CFixedFloat is the arbitrary precision floating point implementation in Kalles Fraktaler 2. I was thinking of the floatexp class.
I use GMP and MPFR (and recommend them too) for high-precision high-range (such as reference orbits), but for low-precision high-range (such as series approximation coefficients and deep orbit deltas) a hardware double+separate exponent is likely to be faster and take less storage than GMP or MPFR. I must admit I haven't actually run benchmarks yet though!
I've commented this before without any feedback so I hope I get it now

When you multiply for example two 100 digit values, you need to put the result in a 200 digit mantissa to ensure correct result. Because, if every digit is 9 the least significant digit may flip all digits and affect the result.
However that small value is meaningless when making the Mandelbrot set, so you don't need more than 100 digits. This is even how fractal extreme is doing it, so even fx is a approximative! That is why it is so fast compared to if you use standard arbitrary precision libraries.
Further, when squaring you actually don't need to use more than 50 digits.
Does a general library such as GMP not make accurate correct calculations? I doubt that...
I have tried different options and using the same type of method described by the author of fx. This requires more memory per digit, and I was able to make it faster than the current up until some 4000 digits. Then it got significantly slower! I recall Botond mentioned this as an explanation why mantel machine doesn't go deeper, there is some optimization in the compilers that is lost.
I've also tried SIMD and was able to get more than 3 times faster. However that is only applicable on double, up to e600. Mandel machine analyses the values of the "delta" and use double, or even float, when values get appropriate high, but it is difficult to determine that in a chaotic system and MM have still some problems with glitches.
So, my conclusion is that I am not able to do anything at the moment
