Annif takes several seconds to start even when it's doing nothing but printing the version number or help text:

$ time annif --version
0.54.0.dev0

real	0m4,398s
user	0m4,322s
sys	0m0,470s

I investigated this a little bit using the -X importtime feature in Python 3.7+ and the tuna tool for visualizing profiling information. It seems that the time is mostly spent importing large libraries such as tensorflow, scikit-learn, optuna, connexion and nltk:

These libraries are all unnecessary in simple operations such as annif --help and --version so it would be better to avoid importing them altogether. There are some tutorials on lazy importing (e.g. this one) and the importlib library contains (since Python 3.5) a LazyLoader utility class that could be used here.

I experimented a bit with this lazy_import function but couldn't get it to work for nltk submodules:

# Adapted from: https://stackoverflow.com/questions/42703908/
def lazy_import(fullname):
    """lazily import a module the first time it is used"""
    try:
        return sys.modules[fullname]
    except KeyError:
        spec = importlib.util.find_spec(fullname)
        module = importlib.util.module_from_spec(spec)
        loader = importlib.util.LazyLoader(spec.loader)
        # Make module with proper locking and get it inserted into sys.modules.
        loader.exec_module(module)
        return module

This needs more experimentation but for now I'm just opening the issue...