I was set some breakpoints in odoo ir.attachment and found most of the time consuming was before the "create" method.
This is the Android code:

HashMap map = new HashMap();
map.put("res_model", res_model);
map.put("res_name", res_name);
map.put("type", "binary");
map.put("res_field","image");
map.put("res_id", res_id);
map.put("name", "image");
map.put("datas", image2Base64(imagePath));
try {    
    models.execute("execute_kw", asList(            
        db, uid, password,            
        "ir.attachment", "create",            
        asList(map)    
    ));
} catch (XmlRpcException e) {    
    e.printStackTrace();
}