I must admit I did not attempt to reproduce the behavior before making my post and I do see the perceived difference between the issues
If I had to guess, when CCleaner loads the Easy Clean mode (either by default or when switched to as a tab if not loaded first), it force reloads its configuration including winapp2.ini (even though it is not used) and does so before it displays its pane (ie. before the ccleaner window is spawned if the easy clean tab is the default), probably blocking the main thread while it does so
If you set the default tab to custom clean, you'll observe the normal delayed availability of the buttons as expected, but when switching to the custom clean, my ccleaner window became non responsive before switching and then once again after it loaded
The best you can do to treat this as an end user is to trim winapp2.ini which is why I suggested it; Given that this seems to be a pretty glaring ccleaner bug, I'd suggest you post it to the bug report sub forum: https://forum.piriform.com/forum/8-ccleaner-bug-reporting/
I'm not a ccleaner developer so I don't have the agency to fix the bug you're describing here, but I think they'd be keen to hear it.
I think ultimately the slow loading of the large ini is the fault here and you'll see it whether it happens before or after the window displays itself; untrimmed ini clearly had the longest delay on my machine, the trimmed ini was still slow to load but noticeably faster, and an empty file just called winapp2.ini caused no delay at all