Yep, I ran into the same render crippling issue with my dome(s) effect with TOS.5 E in modo. It took upwards of an hour+ to render a close-up frame of the dome(s).
Once I got into LightWave with her come TOS.5.2 E, I totally redid the effect (more to my liking as well), but always made sure to figure out a way-less expensive method. Went from hour+ down to like 10 mins for the close-ups.
Best of success with that on your end, V.