I am having the an issue using masks with a mouse click. Using the keyboard shortcuts has no problems.

Lightroom Classic masking tools have a 5–6 second activation delay when selected by mouse, but activate instantly when selected using keyboard shortcuts.

Steps to reproduce:

1. Open a RAW image in the Develop module.
2. Open Masking.
3. Click Linear Gradient with the mouse and draw a gradient. The first mask may activate immediately or after a short delay.
4. Create another mask and click Linear Gradient again with the mouse.
5. Lightroom takes approximately 5–6 seconds before the Linear Gradient tool becomes active. Once active, drawing/applying the gradient is immediate.
6. Instead, use the keyboard shortcut M to create/activate a Linear Gradient. The tool becomes available immediately, even on the same image with the same existing masks.
7. Alternating between the two methods consistently reproduces the behavior: mouse click = ~5–6 second delay; keyboard shortcut = immediate activation.

Additional testing:

• Reproduced on Lightroom Classic 15.3, 15.4.1, 15.5, and 15.5.1.
• Reproduced with RAW and 16-bit TIFF files.
• Reproduced with Linear Gradient and other masking operations.
• Reproduced in a new catalog.
• Reset Lightroom Classic preferences using Shift+Alt at launch; problem remained.
• Tested with Automatically Write Changes Into XMP both enabled and disabled; no difference.
• Tested different physical mice (Logitech and Microsoft); no difference.
• Tested on both monitors; both are 1920×1080 at 100% Windows display scaling; no difference.
• Tested GPU configurations previously; masking delay persisted and GPU/VRAM were not saturated.

Key observation: Lightroom is capable of activating the exact same masking tool immediately when invoked by keyboard shortcut. The delay occurs specifically when initiating the masking tool through the Lightroom UI with the mouse. Once the tool activates, applying the mask itself is fast.

System: Windows 11, Lightroom Classic 15.5.1, AMD Radeon RX 9060 XT 16 GB.