Some problems with many images (cache and timeouts)

Hi,
I extended the default blog item page to create a gallery with all the images from the page.
However initially the post did not have the images.
When I added the 540 images nothing happened and I searched for an error in my script.
Just to give it a try I cleared the cache and voila, now I get the error that processing took more than 30s (I create thumbnails with 300x300 for all images).
However after a lot of refreshs it actually worked.
My questions:

1. Is there a way I can clear the cache only for that post? There is a lot of stuff in the doctrine folder and the site slows down a lot.
2. Can I extend the timeout of 30s?

We currently don't have a way to clear cache for a specific page as this requirement just not really come up before. You can however, clear images only (which is where the thumbs are generated into):

bin/grav clear --assets-only

You can change your 30s PHP processing timeout via php.ini, or perhaps even in your .htaccess.