I have no clue about the programming of this algorithms, but I played yesterday with the tone mapping in GIMP (GEGL) 2.10 RC2 and it was extremely slow:
http://www.gegl.org/operations/tonemapping.html
Especially the Mantiuk 2006 (GEGL has no Mantiuk 08) was nearly unusable with an 8192x4096 image. Perhaps you can port some of you optimisations to the GEGL code.