This does the intended job however most of the time it is the caster who is using it on themselves thus lies the problem, it's not reliable!
A lot of the poison dished out by encounters are fast acting ones so interruptions are very high and some even deals out great amount of damage so the sooner it's cured the better. This spell becomes useless because you can just drink an antidote and rid yourself of all the hassle, heck you don't even have to wait for the cleric to waltz on over, just pass that potion bottle over and cure it instantly and without any chance of failure.
The only advantage Slow Poison has is that you can use it on incapacitated targets but even then it's better to cure their status ailment then have them down an antidote instead. I have a party with multiple characters that has slow poison readily available and I opt out to use antidotes every single time, this is a ridiculous waste of an ability/spell.
Because of the nature of how damage works in this game, the rate of poison tics, and the sheer amount it's encountered throughout the game the casting speed of this spell should be reduced to 0. It's really no different compared to a speed of 1 and it becomes greatly more reliable.
Antidotes has it's own advantages, allowing anyone without the ability/spell to cure themselves and also can be used while silenced. However as it is right now, even if you have the ability/spell it's still clearly the better option.
If anyone played solo, you'll see how useless Slow Poison is. Even if you gain this ability from the chapters or if classed as a cleric and has this memorized, you will never really want to use this to combat against poison as one failed attempt can end your game.