Why so many chase frames? (rhetorical question)

One thing you can do is only take out some chase frames, where it makes sense, i.e. like reducing a baron/hell knight's planting steps to one chase frame, as it would make sense that being large intimidating creatures, they would plant their footsies in a more pronounced (and thereby jerky) manner.

Another great place to get frames is from archvile resurrections. Just note down the length in tics of the resurrection, and reduce the resurrection to a choppy 2-4 frames, adjusting the lengths the match the original timing. I like to make it look like the archvile raises the body for a little bit before suddenly shocking them back to life. You can also get more frames by cutting back on animations that wouldn't otherwise be seen or noticable, like taking all of the item-respawn frames if you don't intend your map to be played in multiplayer or reducing the health/armor bonuses with their needlessly long glowing animations to only a simple 2 frame flash, or by cutting off the ends of death animations where they get far more subtle (like the ends of the baron/hell knight death animations).

I know this didn't really answer your question all too well, but if you don't want to bother with the task of doing trial and error with chase frames, it can help tremendously to look elsewhere first.

If you've already looked elsewhere and done what I've suggested, then... oof. ;~;

The duration of the chase frames affects monster aggressivity, because every call to the chase function has a (somewhat) fixed chance of starting a ranged attack, so twice more delay means (roughly) twice less frequent ranged attacking.

The monster's speed affects navigating tight spaces, because monsters either move a full length of a step, or their movement in that direction fails, so that moving a greater distance per step makes it hard to enter spaces just wide enough for the monster without making a collision that would cause the monster to move in another direction instead.

Similarly, moving a greater distance per step makes it harder for melee monsters to get close enough to their targets to melee attack them, increasing the probability that the player will experience melee monsters walking almost to him, but turning into a different direction right before reaching him, because in their current position, they're still too far to use a melee attack, but in a position one step closer to the player, they would already collide with his body and so they can't move there.

The monster's speed also affects climbing staircases. The rule for monster climbing is that a monster can't move onto a position where its hitbox overlaps (in 2D top-down view) with a sector that has floor height more than 24 units below the height where the monster's feet would be in this position. If a staircase has thin-enough steps relatively to the monster's diameter, the monster can't climb it - unless the monster is fast enough to move beyond the blocking positions in a single step. Generally, the narrower monster diameter and/or the higher monster speed, the steeper staircases it can climb.

To avoid changing monster behavior in the abovementioned ways, you may remove as many chase frames as you want and leave in as few as one frame, but you may not change the duration of the remaining frames or the monster's speed. This pretty much necessarily comes at the cost of changing the visual aspect.

About noticeability, I can tell you right away: Any change will be noticeable. People who play Doom for a long time know the exact timings of every animation, speed of every object in the game, and so on. I've repeatedly seen people claiming that it felt unnatural to them when a relatively long animation's overall length was changed by 1 tic. Even more often, I've seen people complaining about the slightest changes in monster behavior.

The trick is to replace sprites of modded monsters (or weapons, pickups, decorations, whatever) with custom sprites, so that players will feel like getting to know a new monster, not redefining their feel for a monster whose default behavior they're already familiar with in every detail.

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