If you use csv edits:
Try this but instead of spider larva you apply this for ALL bosses in Abyss (look for the boss files). And instead of changing the value 2.7 -> 10, change it to 0 or 0.1 and try it out.
1. Open: 2Moons\data\share\creature\monster.csv
2. On box Z2 change the 2.7 to 10
This will only change mob speed for Spider Larva.
To change the mob speed of other monsters you can do the same steps but for the rows corresponding to those monsters.
0 -> Might not move at all
0.1 -> Slow motion
I haven't tried it yet but from a logical point of view it should work.
If you use CE then refer to this: