StackExchange.Redis and FireAndForget for async/sync operation

I'm trying to use StackExchange.Redis library to implement caching. One thing that I don't understand is how to properly use FireAndForget approach. Based on documentation I see that i have to provide CommandFlags.FireAndForget in case I don't want to wait until operation finishes.

I wrote test console application (net 4.7.2, StackExchange.Redis version is 2.0.601), I have 4 methods that emulates adding stuff to cache by single key:

//Async method with FireAndForget flag:  

public static async Task<bool> AddAsyncWithFF(string key, object entity)
        {
                if (DataBase != null)
                {
                    var redisValue = RedisValueHelper.GetRedisValue(entity);

                    if (redisValue != default(RedisValue))
                    {
                        return await DataBase.StringSetAsync(
                            key,
                            redisValue,
                            expiration.DateTime.Subtract(DateTime.Now),
                            flags: CommandFlags.FireAndForget);
                    }
                }

            return false;
        }

 //Async method without flag
 return await DataBase.StringSetAsync(
            key,
            redisValue,
            expiration.DateTime.Subtract(DateTime.Now),
            flags: CommandFlags.None);

 //Sync method with FireAndForget flag
 DataBase.StringSetAsync(
                key,
                redisValue,
                expiration.DateTime.Subtract(DateTime.Now),
                flags: CommandFlags.FireAndForget);

 //Sync method without flag
 DataBase.StringSetAsync(
            key,
            redisValue,
            expiration.DateTime.Subtract(DateTime.Now),
            flags: CommandFlags.None);

I create 18 mb of random data and save per one key for testing purposes, here is the result (time measured using StopWatch class):

• Put sync data. Elapsed 229 ms
• Put sync with FireAndForget data. Elapsed 234 ms
• Put async data. Elapsed 12928 ms
• Put async with FireAndForget data. Elapsed 4508 ms

What I don't understand:

• Why sync calls with FF flag and without take the same time? It seems this flag is not in effect for sync calls.
• Why async method with FF flag takes longer than sync method?
• What is more interesting is that fact that async FF version takes 2-3 times less to execute than async without FF flag...

Ultimately I'd like to understand why can't I have asynchronous call with Fire and Forget flag that takes 200ms?

Thanks.