Transition between photos is very slow

Lots of complaints here about sluggishness in LR 15, and there are probably multiple causes. I don't know which magic spells you've tried, but here's a list of ones that have helped some others. Restart your computer after trying each step:

- Try updating your graphics driver by going directly to the manufacturer's web site, rather than relying on Windows Update or a manufacturer's update utility:
https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html#solution-4

- In Preferences > Performance, set:
. Use Graphics Processor: Custom (rather than Auto)
. Check Use GPU For Display, Use GPU For Image Processing, and Use GPU For Export
. Use GPU for Preview Generation: On (rather than Auto)

- Make sure you catalog folder is not being synced to a cloud service like Google Drive, Onedrive, Dropbox, Icloud.

- Set Preferences > Performance > Enable HDR in Library to On. A year or so ago, that helped with sluggish filmstrips, though I don't recall recent reports about it. But it's harmless.

- Do File > Optimize Catalog.

- Temporarily disable all anti-malware / anti-virus packages, including Windows Defender. If that helps, then add exclusions for the catalog folder and all photo folders.

- Disable Catalog Settings > Automatically Write Changes Into XMP.

- Disable the options Catalog Settings > Metadata > Automatically Detect Faces, > Analyze Photos Of Selected Source, and > Automatically Analyze All Photos In Catalog.

- Try resetting LR's preferences:

https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/

LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.

- Do Preferences > Performance > Camera Raw Cache Settings > Purge Cache.

- Delete the entire preview cache by deleting <catalog>Previews.lrdata from the catalog folder. LR will rebuild them incrementally as you browse your library, or you can select all photos and let the command Library > Previews > Build Standard-Sized Previews run all night.