Long story short, I'm writing some mod which allows you to increase your arrow speed and bow draw speed.

Currently I'm handling this though events.

    @SubscribeEvent
    public void onArrowSpeed(@NotNull EntityJoinLevelEvent event) {
        if (event.getEntity() instanceof Arrow arrow) {
            if (arrow.getOwner() instanceof ServerPlayer serverPlayer) {
                serverPlayer.getCapability(PlayerCapabilityProvider.PLAYER_CAP).ifPresent(capability -> {
                    if (capability.isUnlocked(serverPlayer, ARROW_SPEED)) {
                        int level = capability.getLevel(serverPlayer, ARROW_SPEED);
                        //Level is value between 0 and 20;
                        if (level > 0) {
                            float modifier = 1.0F + level * 0.05F;
                            arrow.setDeltaMovement(arrow.getDeltaMovement().scale(modifier));
                        }
                    }
                });
            }
        }
    }

    @SubscribeEvent
    public void onBowDraw(@NotNull ArrowNockEvent event) {
        if (event.getEntity() instanceof ServerPlayer serverPlayer) {
            if (event.getBow().getItem() instanceof BowItem) {
                serverPlayer.getCapability(PlayerCapabilityProvider.PLAYER_CAP).ifPresent(capability -> {
                    if (capability.isUnlocked(serverPlayer, BOW_DRAW)) {
                        int level = capability.getLevel(serverPlayer, BOW_DRAW);
                        //Level is value between 0 and 10;
                        if (level > 0) {
                        //I have used AT to make this field public from protected;
                            serverPlayer.useItemRemaining -= level;
                        }
                    }
                });
            }
        }
    }
}

Problem I have is this delta movement data is not syncronized with the client, because when I shoot the arrow, at first the arrow goes in another direction and later actually syncs to it's real position.

In other words client-side animation is bugged/not properly synced.

To better visualize the problem I'm including a video:

https://streamable.com/x741gp

Any ideas how can I fix this?

Try setting

arrow.hasImpulse = true

after you change the velocity.

If you look at ServerEntity.sendChanges() that should cause it to send an update packet instead of waiting 20 ticks (the arrow's updateInterval).

Already tried that, unfortunately it doesn't work. Also tried setting field arrow.hurtMarked = true; , but that also doesn't work.

I think your problem is caused by the ClientBoundSetEntityMotionPacket which clamps the maximum delta at +/- 3.9d

Obviously, the same clamp doesn't happen to the real data on the server so there is an inconsistency.