Consider the following TeX document:
----
\documentclass{article}

\usepackage{bbm}

\begin{document}
$\mathbbm{1}$
\end{document}
----

When compiling with pdflatex, the font files for the bbm font are created in the local cache (~/.texlive/texmf-var/fonts) only if they do not exist. When the fonts do exist in the cache, they are used without warning.

When compiling with lualatex, the font files are created every time. When the fonts exist, this results in a simple warning ("mktexpk: [...] already exists."). Trying to create the fonts slows down compilation noticeably.

I am not sure whether this is a misconfiguration of texmf.cnf (or maybe texmfcnf.lua), or a bug in lualatex/kpathsea.