Below is a transcript of a "smallest script" which runs in 328 ms total on my machine. And that includes loading an ubuntu OS image, which could be optimized depending on what dependencies the script needs.

Of course, if you're invoking it on a remote cluster, there are many reasons it can talk longer, especially if the cluster has to scale to satisfy the request. But all those reasons are nothing to do with containers specifically - it's things like cluster capacity, node provisioning time, container pull time, network latency, etc. If you architect things properly, you can get the number below + network latency even for a remote cluster invocation.

    $ time docker run ubuntu echo hello world  
    hello world  

    real    0m0.328s  
    user    0m0.011s  
    sys     0m0.010s

That is not normal. Small scripts should launch in milliseconds, not several seconds.