A big contributing factor to the massive laggy keep and fortress battles is the graphical effect of the tank m4 ability, Immaculate defence.
This ability puts up to 6 animated *stacking* transparent texture layers per party, with up to dozens per faction.
The stacking transparency layers of this effect are most likely the biggest culprit to the FPS drops on this dated engine.
If the material/texture on the effect we're changed from transparent to cutout, opaque, or some other solution to make them not stack such as displaying a different effect for those outside of your group, it would probably significantly improve performance in huge fights and important moments when dozens of players are getting sub 5~ fps.
Also the sheer number of actions and movement in such a confined space it also a part of this, but forts are playable with acceptable FPS in almost every moment except for the m4 spam.
Ofc turning off spell effects for everyone already solves this, but this would most likely fix the FPS lag bombs for those who play with all spell effects on, and streamers
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