How to trace the cause of lag in Windows?

You want to use PerfView in order to spot the bottlenecks in a process without having the code, ie., zoom in your case.

You can do this too with Intel Advisor.

Few suggestions to resolve the lagging:

Open the settings of the Zoom app and change the following settings under the individual sections described:

Video:

• Change Aspect Ratio from ‘16:9’ to ‘Original’
• Uncheck ‘Enable HD’
• Uncheck ‘Mirror my Video’

Uncheck ‘Display up to 49 participants per screen in Gallery View’

Audio:

• If you have a headset with a good microphone then:
• Go to Advanced Settings
• Select ‘Disable’ under the Suppress Persistent Background Noises’

Virtual Background:

• Disable the use of virtual background if at all possible.
• While in a call:
• Make sure to stay in speaker view instead of gallery view. This has the least amount of impact but does make a little bit of difference.

(from rahisystems)