AI Denoise not applied on import, photos processed only when visible, severe performance issues

"Browsing photos is laggy and switching between images in Develop can take up to 5 seconds per photo"

There are many posts about LR 15 being very sluggish moving photo to photo in Develop and sometimes Library Loupe. There may be multiple causes with similar symptoms. See here for a compendium of magic spells that have each helped at least a couple of people:

https://www.lightroomqueen.com/community/threads/magic-spells-for-sluggish-lr-15.54013/

"After import, images are not fully processed in the background. Lightroom only processes photos that are currently visible in Grid view. For example, when importing around 10000 photos, only about 300 visible images get processed. I have to scroll through the grid to trigger processing of the remaining photos. ... Thumbnails still show the three dots icon"

I think by "processed" you're referring to LR's building previews for the imported photos.  The three dots in the upper-right corner indicate that preview building hasn't completed yet.

It may be that LR is unusually slow building the previews in background, or it may be that it got permanently stuck. You could let LR sit overnight and see if it finally computes all the previews for an import. That would provide a little more information about what's going wrong.

As a workaround, you could select all the imported photos and do Library > Previews > Build Standard-Sized Previews.

"When I select an import preset with AI Denoise enabled, the denoise is not applied. After the import finishes, I have to manually run AI Denoise on each photo."

Building on Johan's reply, if you look at the photos after import in Develop, the AI Edit Status button in the upper-right will be yellow, indicating the AI setting was applied but not computed.

After the import completes, select Previous Import in the Library Catalog panel, select all the displayed photos, and do the menu command Photo > Develop Settings > Update AI Settings. That avoids the need to update each photo individually.

When I select an import preset with AI Denoise enabled, the denoise is not applied. After the import finishes, I have to manually run AI Denoise on each photo.

By @Fafasha

That is as designed. AI options such as denoise and applying the adaptive profile, will be enabled but not yet calculated if you apply these through a preset on import. This is to speed up the import process itself.

Unfortunately you don't have gave us any informations about your system. So all proposed solutions are a guesswork.

Please provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

Is your system up-to-date? Do you have installed all recent updates and patches for the operating system and the device drivers, especially the graphic driver. Please make also sure that you have installed the Studio version and not the Game Ready version of your NVidia driver.