Update Lightroom Classic to the latest version.
Live preset preview in the Loupe view causing system slowdown
When you move the mouse pointer over any preset in the Presets panel (Develop module), Lightroom Classic displays its live preview in your photo in the Loupe view. The live preset preview feature─introduced in the April 2018 release of Lightroom Classic CC v7.3─is slowing down your computer's performance.
Product version affected
Lightroom Classic CC 7.3
Windows and macOS
In Lightroom Classic CC 7.4, a new preference option has been provided that allows you to disable the live preset preview feature. To do so, follow the instructions below:
Launch Lightroom Classic and choose Help > Updates. For related information, see Keep Lightroom up to date.
After updating to the latest version, perform the next steps.
Choose Edit > Preferences (Win) or Lightroom > Preferences (Mac).
In the Preferences dialog, go to the Performace tab.
In the Performance tab, deselect Enable Hover Preview Of Presets In Loupe.