Not sure how I feel about this. On one hand, having high speed is the one thing that makes the game incredibly easier for every class, so penalizing it in some way feels justified. On the other, so does reducing energy cost—which isn't penalized in any way and is far more effective as well; a pair of blessed 7LB alone provides an endgame-worthy equivalent to movement speed increase, and there are also class powers that match or outperform it (not to mention you can stack both).

It occurs to me that to make things more fair, either both speed and energy cost should affect XP gain in some way, or neither of them. And in both cases net change in XP gain should be equalized with the current state, so the typical progression doesn't feel significantly different. Individual penalties for either parameter, in case both are penalized, would be lower—this would by extension make the Slow Monster abuse less rewarding.

Movement energy cost is different from melee attack energy cost, so it doesn't necessarily make melee fights easier. For ranged and casters though it's another matter. It would make sense to give a different penalty for different fighting styles but that gets so complicated.

As for speed I don't like that you can have speed-boosting gear equipped when fighting someone like Nuurag-Vaarn, and even if the fight lasts 50 turns you can still take them off for the final blow to maximize your xp. It's a bit silly.

No i dont like this. Having to get enough xp for an ultra is very hard sometimes and slow monster helps cut down on tedious grinding to get XP. Its always a risk to slow yourself even if they wounded they can rage. Slowing yourself makes you very vulnerable, and you should be rewarded for doing so.

The system is good the way it has always been. Penalty on XP for speed, and bonus XP for slow.

While changing the XP reward according to the speed of the PC always made at least some sense to me, the punishment of losing 50% of the XP if you actually use Slow Monster in the most obvious way possible seems overly punishing. Slow is hardly the most debilitating status effect - confusion, blindness, stun, and paralyzation are all much more potent - yet it is the only one that inflicts this XP penalty. I can only wonder how confusing this might be to a new player who happened to start with a spellbook of Slow Monster and keeps wondering why their XP gains are lacking.