Unity shader optimization

I have a problem with badly optimized palette cycling function of the background shader:

Shader "Background/Earthbound"
{
Properties
{
    [Toggle] _Blend("Blend?", int) = 0

    [Header(Texture A)]
    _TexA ("Texture", 2D) = "white" {}      // ensure "Repeat" wrap mode
    _PaletteA("Palette Cycle", 2D) = "white" {} // ensure "Clamp" wrap mode
    [Enum(None,0,Horizontal,1,Interleaved,2,Vertical,3)] _OscillationVariantA("Oscillation Variant", int) = 0
    _ScrollDirXA("Scroll Direction X", float) = 1
    _ScrollDirYA("Scroll Direction Y", float) = 1
    _ScrollSpeedA("Scroll Speed", float) = 0
    _OscillationSpeedA("Oscillation Speed", float) = 1
    _OscillationAmplitudeA("Oscillation Amplitude", int) = 32
    _OscillationDelayA("Oscillation Delay", int) = 1

    [Header(Texture B)]
    _TexB("Texture", 2D) = "white" {}
    _PaletteB("Palette Cycle", 2D) = "white" {}
    [Enum(None,0,Horizontal,1,Interleaved,2,Vertical,3)] _OscillationVariantB("Oscillation Variant", int) = 0
    _ScrollDirXB("Scroll Direction X", float) = 1
    _ScrollDirYB("Scroll Direction Y", float) = 1
    _ScrollSpeedB("Scroll Speed", float) = 0
    _OscillationSpeedB("Oscillation Speed", float) = 1
    _OscillationAmplitudeB("Oscillation Amplitude", int) = 32
    _OscillationDelayB("Oscillation Delay", int) = 1
}
SubShader
{
    Tags { "RenderType"="Opaque" }
    LOD 100
        ...
        ...
        ...
        // palette cycling (too expensive right now...)
        float4 paletteCycle(float4 inCol, sampler2D paletteCycle, float paletteCount)
        {
            float4 outCol = inCol;

            int paletteIndex = -1;
            for (int i = 0; i < paletteCount; i++)
            {
                if (inCol.a == tex2D(paletteCycle, float2(i / paletteCount, 0)).a) // match alpha values (greyscale)
                {
                    paletteIndex = i;
                }
            }
            if (paletteIndex >= 0)
            {
                int paletteOffset = (paletteIndex + _Time.y * 12) % paletteCount;
                outCol = tex2D(paletteCycle, float2(paletteOffset / paletteCount, 0));
            }
            return outCol;
        }
     }

I use 2 grayscale sprites for the background animation - main bg (256x256) with "Repeat" option and palette (17x1) with "Clamp" option.

How can I optimize it?

Unity Version: 2020.