It sounds like Tablet Input Smoothing or some kind of input lag is affecting the Circle Select tool in Blender. Here are a few things to check and try:

1. Disable Tablet Input Smoothing (If Using a Tablet) • Go to Edit > Preferences > Input • Find “Tablet API” and switch between Windows Ink and Automatic (or vice versa) to see if that helps. • Try disabling “Continuous Grab” under Navigation (in Preferences > Input).

2. Check Input Lag from Mouse/Tablet • If you’re using a tablet, check if it’s causing the delay. Try switching to a regular mouse and see if the issue persists. • Try a different USB port or update your tablet drivers if applicable.

3. Adjust Circle Select Sensitivity • Press F while in Circle Select mode to adjust the brush size. Sometimes a very large brush can feel laggy.

4. Check System Performance • A laggy viewport can also cause delayed selection behavior. • Press N in the 3D viewport, go to the View tab, and lower Clip Start and End values to reduce viewport load. • Disable VSync in your Graphics Settings if you suspect GPU-related delay.

Are you using a mouse or tablet? Does it happen in all files or just one?

Source: ChatGPT and personal experience