I would really recommend you to pre generate previews as per system cron job.
@daily /usr/local/bin/nextcloud-preview.sh #Nextcloud preview generation
#OR
@daily php /var/www/nextcloud/occ preview:pre-generate
This will save you CPU time in peak hours. Just execute it at the evening to be ready for a next day.
You can always interrupt it and as you say, it uses only 1 Core, should not be too bad for your performance.
For my old 2 core Cubietruck
For my new Ryzen with 16 GB RAM around 6 Hours for the same amount of job after migration to the newer Server.
Even after I moved out Preview folder to the SSD, by simply mounting it to the nextcloud folder as preview app, now All Nextcloud files are on a huge HDDs, but previews are on a SSD that makes random access to them extremely good.
mount | grep preview
/var/previews on /mnt/raid/nextcloud/data/appdata_xxxxxxxxxxx/preview type ext4 (rw,relatime)
Bunch of PDF Previews are loaded in 200-600 ms
