Exhaust Timings

If I exhaust a champion WHILE they're casting a spell, does it reduce the effectiveness of that spell?

Some considerations: Jinx casts ult, I exhuast, ult hits. Or Ziggs, etc. Upon impact is the damage reduced or does the timing consider upon-release?

What about channeled abilities, like Malz's or Kat, if the ult has STARTED does exhaust reduce it?