You should expect long generation times when using this theme (~25s for all templates on a Raspberry Pi 4B with 15 months of data). If you are getting into trouble because of this you can comment out the stale_age options in skin.conf at the templates section, eg. for the years page:

[[[year]]]
    template = year.html.tmpl
    stale_age = 3600 # Every hour

This will generate the year.html page only once an hour. stale_age is in seconds, see https://weewx.com/docs/customizing.htm#CheetahGenerator. You can experiment with the stale_age options to find a good balance between being 'up-to-date' and reasonableness.

statistics.html stale age is 43200 (twice a day) by default because it's the most performance-expensive template to generate.

More info about this is available here https://github.com/Daveiano/weewx-wdc/issues/14