F-Stop has some features like Smart albums, tags, ratings, etc…that requires it to keep a lit of all images in it’s own database. Most galleries uses Media Store to get list of images. For features implemented in F-Stop this is simply not enough. So, F-Stop constantly needs to keep list of images on your device fresh. That requires getting list of new images, comparing that list to the list in it’s own database, and adding new and removing missing images.

There are different ways how F-Stop searches for images. It either uses “Custom scanning”, which will read all folders and files in it and check for images. This can be quite slow when you have a lot of images. There is no way around it, as Android intentionally slowed down scanning through years with new file APIs (which is documented on various websites with benchmarks).
Other way is to use “Media Store” which can be faster. You can change this in Settings//Main//Included folders.

However, there are other settings that may affect it, like “Check for changed images”. This setting will (on each scan) check whether date of some images changed, so that it can reread it’s metadata and thumbnails. This is painfully slow when you have a lot of images. Not sure why, but getting last modification date for a file is relatively slow on Android, now multiply that by number of images and it can take a minute or more.

So, I would first try changing those two settings mentioned above (scan type and check for changed images (if it is enabled)).
If this does not help, how many images do you have?