Ahh ok, I thought it was setting the uniforms that was the heavy part. My palettes are pretty small, only a 3-4 colours per row, and maybe 6 columns max.
If Vertex batche isn't a good measure then I think my issue might just be the tablet I tested it on was super weak. I've not checked what the spec is, but I can't imagine an old MS Surface is very powerful! (it did crash trying to play Sonic Mania lol)
Thanks for your help!