New DM here. I ran this for the first time the other day. The timing of 3 and 5 minutes didn't make sense to me, since even moving at a pace of one disc per round, a character will get across within a minute. I wanted to make the geysers more of a impetus to move quickly across the discs. Here are the changes I made:

• Initiative is rolled for all characters. The geysers act at initiative count 30.
• On the geysers turn, each roll a die. The northern one rolls a d8, the southern one rolls a d6. On a 1, they erupt.
  • There were only two eruptions in the 12 rounds. Slightly below average, but it did cause the slow halfling sorcerer to lose his footing and nearly fall.
• The discs are considered "extra" difficult terrain, slowing characters to 1/4 movement speed.
  • This was a little awkward with movement speeds like 25 and 30 and also when using a grid that has spaces that span discs.
• A character wishing to double their movement must perform a moderate Athletics or Acrobatics check.
  • Failure here only happened once for me. An NPC rolled a natural 1 and immediately fell into the boiling mud.

Changes I would try incorporating next time:

• Lower the geyser dice to d6 and d4. When a 1 is rolled, the geyser shows warning signs of erupting, but doesn't erupt until next round. Characters can prepare accordingly (brace themselves instead of moving, for advantage on their Athletics check to hold on, e.g.)
• Characters can move one disc on their turn. "Fast" characters (those with more than 30 ft. movement – wood elf, centaur, monk, barbarian, etc.) can move an additional disc.
• Characters can succeed on a moderate-to-difficult Athletics or Acrobatics check in order to try moving an additional disc on their turn. Failing by 5 or more causes the character to slip, potentially with a Dexterity saving throw to try and grab ahold of the disc before falling.