Here's a list of timers that exist:
Poison
Gradual Petrification
Count/Doom
Reflect/Wall
HP Leak
Stop
Regen
Paralysis
Sleep
Stop affects the following status timers:
Regen (healing becomes a constant in the SNES version)
Paralysis (timer stops - there are bugs in the SNES version when used with this status)
Sleep (timer stops - there are bugs in the SNES version when used with this status)
Stop (resets the Stop timer)
HP Leak (timer stops - there are bugs in the SNES version when used with this status - only affects characters though, enemies are unaffected)
Gradual Petrification (timer stops - there are bugs in the SNES version when used with this status)
Count/Doom (timer stops)
Poison (timer stops)
Reflect (timer stops)
Haste affects the following status timers:
Gradual Petrification (time to petrification is accelerated)
Regen (number of healing intervals is reduced)
Stop (time to being released from this status is reduced)
Poison (time to take poison damage is accelerated)
HP Leak (timer is reduced from 60 to 48, then 44) - note that haste is +3, so the timer adjustment is 4 HP
Reflect/Wall (time before status removal is reduced)
Summary: Haste accelerates most timers.
Slow affects the following status timers:
Gradual Petrification (time to petrification is slowed)
Regen (number of healing intervals is increased)
Stop (time to being released from this status is reduced)
Poison (time to take poison damage is slowed)
HP Leak (timer is increased from 60 to 90, then 120) - note that slow is -8, so the timer adjustment is 50% slower, then 100% slower...
Reflect/Wall (time before status removal is increased)
Summary: Slow decelerates most timers.
Edit: This list should be mostly complete, unless I forgot something major.
Second Edit: Additionally, all statuses other than Sleep can be reinflicted to reset/update/change the timer.
Third Edit: I totally forgot about the Mage (Mind Flayer summon).
4th Edit: Some summary updates are added to make the info more complete.