When using fireworks rockets, culling processing slows down the flight speed of both the server and client's elytra.

This doesn't make a whole lotta sense.

EntityCulling doesn't touch anything on the server. Having it installed there does nothing. The mod works on the client only, preventing the rendering of entities and block entities that are not visible, most likely due to being behind a wall. The mod doesn't and can't stop the logic from ticking.

Firework rocket boosts are calculated on the server side, as is most logic. This is noticeable if a server lags while you are trying to firework boost while elytra flying; it just wont boost you until the server catches up.

For the record, the firework boost actually happens clientside and caused by the rocket entity for whatever reason. Thats one of the reasons they are allowed to tick, even when culled in the provided default config: https://github.com/tr7zw/EntityCulling/blob/main/EntityCulling-Versionless/src/main/java/dev/tr7zw/entityculling/versionless/Config.java#L23

Fireworks are in the default tick culling config, don't take them out for this exact issue.