Showkase has a lot to do on publishing a site (for example building page records and generating thumbnails for gallery index pages) and, with a large site, this can take some time.
Unfortunately, there is little that a user can do to reduce the publishing time.
Work was done in Showkase v1.7.3 to minimize the publishing time. Please see the Version History page where you'll see this entry for Showkase v1.7.3:
Faster publishing for sites with large numbers of galleries
At the time, I created a test site with 128 galleries.
Publishing after changing the theme or the value for 'Customize Theme -> Gallery Index Page -> Thumb percent height' (which requires new thumbnails to be generated) took approximately 60 seconds.
However, subsequent publishes (without making any changes that would affect the thumbnails) took approximately 5 seconds.
Previously, thumbnails would always be generated and the publishing time for this sample site would always have been approximately 60 seconds.
If you are using a version of Showkase prior to v1.7.3, then please try upgrading to the latest version (v1.7.4) and you might see a difference in the publishing times.
For reference, full instructions for upgrading a Showkase site can be found here.
With Showkase v1.7.4, try changing the theme (for testing purposes... you can change it back afterwards), publishing your site and then try publishing it again immediately afterwards (without making any changes).
The subsequent publish action should be significantly faster than the initial publish action.
I realise that you will not be publishing your site without making any changes but this should at least check that the thumbnail generation is not happening when it is not necessary.
If you are already using Showakse v1.7.4, then, unfortunately, there may be little than can be done to speed up the publishing on your web server.
Please check with your web host to see what version of PHP your web server runs.
If you are using PHP 5, then try upgrading to PHP 7 (if your web host has this available for your hosting plan) to see if this results in a performance boost.
It might also help to know exactly how large your site is so please let me know how many pages of each type you have within your site and approximately how many images you have in each gallery page.
Thank you.