The bug

If you set chunk builder to Fully Blocking, placing a light source will cause significant lag spike. Even though it is one of the things controlled by this setting, it does seem it is reasonable to better this, especially for the initial huge lag spike.

How to reproduce

1. Place a light source down, while being on Fully Blocking chunk builder setting. Enabling chunk borders and frame time graph is recommended.
   Result: A noticeable lag spike occurs.
2. Continue to break and place said light source.
   Result: More smaller lag spikes appear. Those wouldn't appear in Semi Blocking setting.
3. Travel to another chunk a few chunks away from the one you were placing light sources in in step 1.
   Result: The result of step 1 is repeated.

Expected result: All outcomes involving a lag spike would not be the case.

Additional info

The lag spike from placing the light source for the first time in a chunk still occurs in Semi Blocking, but it is significantly less severe. Leads me to believe this issue exists no matter the setting and Fully Blocking just reveals it the most.