Problem goes away when disabling the addon,
specs
5900x ryzen
3080 nvidia rtx
128gb 3600 mhz ram
corsair mp600 1tb ssd

From what I know, this is no longer a problem. Please report back, if it still is.

Seems to be solved as far as I am aware.

Thanks for the fast response @btnmasher 👍🏻

I got same issue starting with classic tbc and Questie-v6.3.11-TBC version. Questie addon causing this issue.

Same here, please, someone fix this.

The main issue lies on Blizzards side.

With the recent release of v6.3.12 we introduced a workaround for this which should help a lot!

Please update to the latest Questie and report back.

For future reports please add a screenshot of the Profile after you experienced these lags.

You can open the Profile in the Advanced section of the Questie settings!

A few seconds later:

Immediately after a huge lag spike:

Oh to be clear @mkavalecz the Profiler only records while it is open (at this point). So you have to have it open while you are testing the Shapeshifting 🙈

I usually just drag it down in one corner and let it sit there partially off screen until i need it.

Ah, sorry. I thought that might be the case, but I didn't want to have a huge window open in combat.
For me, this usually happens in combat, when I try to reproduce it by just shifting forms and moving around, it usually works okay. Anyway, I'll try to get back with some better results. The update did seem to help a tiny bit, but it's still terrible.
I'll try moving the window away as @drejjmit suggested.

It's much better but sometimes I got a second lag when shapeshifting to cat form. I don't get same behavior with other questing addon like classiccodex, codexlite, carbonite etc. only happens with questie addon.

Adding a +1 to this. I am on the latest update and it hasn't made it any better. It gradually worsens until it is particularly bad to the point of multi-second-long freezing. I can /reload to help it for a while but only then it's a temporary alleviation. Made much worse since part of the Feral rotation involves powershifting which makes it unplayable with this addon enabled.

@sungerbob @btnmasher Please read my comment above. We need data of our Profiler to investigate this further. Please start it up and then provoke the issue you have.

I'm having the same issue. It gets worse the more time goes on. Here is the outputs of about 1 minute of collection.

10 minute data collection on a druid with combat included.

After further investigation we have to say that this is NOT A QUESTIE BUG, even though Questie increases this issue further. Please report it to Blizzard.

We will still try to implement a fix/work around for it! 👍🏻

How should we report it? Can you share some details? I'm more than happy to do it, but I can't just say that "Shapeshifting causes lag, and if I enable an addon, it gets much worse."
Is this related to the issue that changing tracking (for example track humanoids) causes a lag for a lot of people?

Yes @mkavalecz this is related to the "Track Humanoid" spell that automatically gets activated when shifting into cat.

There is a Reddit post of it. In the past "viral" posts there got some attention.

There are also multiple posts on the Blizzard Forum where you should raise attention to this issue:

https://us.forums.blizzard.com/en/wow/t/druid-cat-shapeshift-broken/971944/10
https://us.forums.blizzard.com/en/wow/t/druid-shapeshift-track-humanoid-bug-now-w-hd-video-support/972063/2

Thank you!