iOS Metal Swift - Terrible performance using texture array in fragment shader - ios

Question

I'm making a sprite batcher that can deal with more than one texture per batch. A sprite's data is stored into this huge uniforms buffer which is sent to the GPU as soon as the sprite batch is all filled up. I tried assuming there would be 16 textures, some of which were going to be unused, and based on the textureID sent through the instance uniforms the fragment shader would pick the right texture to use. This yielded in roughly 60 fps with 800-1000 sprites on an iPhone 5s. I then tested this with a single texture and received a satisfying 2000 sprites at 60 fps. Knowing I would still need to be able to swap textures, I decided to use texture arrays to bind one texture with 16 slices. If I render using texture index 0, the fps is just as it was with the single slice texture. Once I delve into further slices, however, performance drops massively.
Here is the shader:
struct VertexIn {
packed_float2 position [[ attribute(0) ]];
packed_float2 texCoord [[ attribute(1) ]];
};
struct VertexOut {
float4 position [[position]];
float2 texCoord;
uint iid;
};
struct InstanceUniforms {
float3x2 transformMatrix;
float2 uv;
float2 uvLengths;
float textureID;
};
vertex VertexOut spriteVertexShader(const device VertexIn *vertex_array [[ buffer(0) ]],
const device InstanceUniforms *instancedUniforms [[ buffer(1) ]],
uint vid [[ vertex_id ]],
uint iid [[ instance_id ]]) {
VertexIn vertexIn = vertex_array[vid];
InstanceUniforms instanceUniforms = instancedUniforms[iid];
VertexOut vertexOut;
vertexOut.position = float4(instanceUniforms.transformMatrix * float3(vertexIn.position, 1.0), 0.0, 1.0);
vertexOut.texCoord = instanceUniforms.uv + vertexIn.texCoord * instanceUniforms.uvLengths;
vertexOut.iid = iid;
return vertexOut;
}
fragment float4 spriteFragmentShader(VertexOut interpolated [[ stage_in ]],
const device InstanceUniforms *instancedUniforms [[ buffer(0) ]],
texture2d_array<float> tex [[ texture(0) ]],
sampler sampler2D [[ sampler(0) ]],
float4 dst_color [[ color(0) ]]) {
InstanceUniforms instanceUniforms = instancedUniforms[interpolated.iid];
float2 texCoord = interpolated.texCoord;
return tex.sample(sampler2D, texCoord, uint(instanceUniforms.textureID));
}
Everything is working exactly as expected until I use a texture slice greater than 0. I am using instanced rendering. All sprites share the same vertex and index buffer.