Do you by chance remember which enemies you were facing at the time when you experienced this? Beholders? Vol worshipers? Lich undead? The artificers and Golems?
J1NG
:: Edit ::
I did find the Plants can cast Spike Growth, where if you fail your Reflex Save, it initiates a Slow effect on you. Currently in middle of testing to see if that's the Slow you're looking at.
:: Edit ::
OK, so the Icons used there are not the typical Slow icon, so it's not them. Shouldn't be thr plant grouping then. Continuing to look.
:: Edit + Further ::
Also just did a PvP test, and the Slow spell is countered by a Haste spell. Grabbing a Haste pot to check, but expecting it to work. :: Further :: And confirmed that the Haste potion did remove the Slow spell. Suspect now that the Slow effect is one of the non-Slow spell effects that are not countered by Haste being re-applied over it. Given the slayer region, possibly Chaos Hammer (but not found an enemy casting that yet), a Golem slow effect, and possibly some others I'm forgetting right now. Will continue checking.