How to diagnose a slow plugin loading?

[pxeger]

When opening nvim, the first time I enter insert mode, there is a noticeable ≈0.5s delay. This doesn't happen on any subsequent entries of insert mode. I presume this is because there is a plugin being lazy-loaded on InsertEnter which takes a while to load. But how can I work out which of my plugins are being loaded on that event, so I can then try and speed that plugin up?

[folke]

open a file, check :Lazy profile tab and see whats at the bottom.
Close lazy, and enter insert mode.
Check Lazy profile again to see what was causing the slow down