Yesterday I had an unintentional test with a friend of mine who had
a Rock Salt Bomb with Low Charge Time Reduction at level 3 vs Ionized Salt Bomb level 10 with Maximum Charge Time Reduction

and this is the result

A 4* Ionized Salt Bomb with [Maximum Charge Time Reduction]'s charging time is as fast as, if not, slower than it's predecessor 3* Rock Salt Bomb [Low Charge Time Reduction]'s charging time.

We did it a number of time and yes, that is the overall result.
I do hope for a change in this mechanics as a maximum charge time reduction should charge faster than a low charge time reduction bomb of the same line and, not to mention, it's predecessor.