Very intriguing! I’ll see if I can take some time out of my day to reproduce this and figure out what the cause is. My suspicion is it could still be rendering issues, which is a bit out of my ballpark, but would be great to at least know why.
Edit: Ah yeah, I’m able to reproduce. Looks like rendering woes. We’ll get a ticket internally for it.
It shouldn’t, but we’re looking into that too as a pain point. We’re having some issues reproducing at the moment, does this happen every time? Like if you ungroup then group the same parts, it’s still slow? Does this also happen when you group as a folder?
One issue that may be causing performance issues when ungrouping groups is that SelectionChanged is fired x amount of times for x number of instances in the group. For example, ungrouping a group of 800 parts will fire the event 800 times. This can be detrimental to any built-in or developer-made plugins that listen to SelectionChanged. As a developer, I would expect SelectionChanged to fire once with every instance selected that was just grouped together.
Years ago I filed a bug report on it, but it was never followed up on. It appears that the issue was partially fixed as SelectionChanged now only fires once when grouping a model. However, the issue still persists when ungrouping models and can be reproduced in the latest version of Studio.
Great find–this is definitely a good lead to follow, a lot of the work done to optimize drag select was minimizing the impact of an individual SelectionChanged.
Have you also tried fixing trying to change sound properties? For me, even when the sound is not playing, changing sound properties is very laggy for some reason. It’s worse when the game is running from what I remember.
Thanks for this update though!
Great update. I’ve noticed an extremely noticeable difference almost immediately when box selecting.
This is excellent, I can really feel how significant the speed up is on my machine, and I’m very happy about it! I can now select thousands upon thousands of parts at once, and I don’t even feel any bump at all anymore.
Great work! 
@VSCPlays and @OneEDM - the post is now updated so that anyone can file a bug! Thank you for calling this out.
Thank you so much! I appreciate this being looked into as it can be a serious drain on performance when ungrouping models for any plugins listening to SelectionChanged.
Performance improvements are always appreciated 
I started experiencing what I think is a bug:
(Trying to drag multiple parts unexpectedly stops moving the parts. Releasing the mouse button and selecting the parts again drags the parts as expected but with an offset from the last drag that broke.)
I’ve been able to recreate this very consistently with a selection of multiple parts (2, 3, 4, 8, etc). I can not reproduce it with a single part. (Edit: In Zomebody’s video this also happens with a single part.)
The multi-part drag seems to break specifically when the mouse is moved quickly. It seems to have the potential to break per drag: some drags can break and some drags will never break no matter how fast or wildly the mouse moves (doesn’t depend on selected parts).
(Macbook with M1 chip)
(Noticed this first earlier today, 2/3)
I think this is a good update, because no one want’s to wait a while just to look at models and other stuff, so W for Roblox!
GOATED update definitely thank you developers.
Now if only you’d release the streamingEnabled update that would let me actually use it…
This is a great update for the experience that I’m working on as it involves MANY parts.
10/10!
I am experiencing the same bug as well (on Windows). I have a heavy suspicion it is tied to this update, as it started happening within the last 24 hours for me.
In my case I also consistently observe the bug with one object selected (see video below). As demonstrated in the video, the dragged object randomly loses focus if you start dragging it shortly after copying and selecting it. I am still holding my mouse button down as soon as it loses focus. The dragging handles also seem to be misaligned, which may be a helpful indicator when looking into this issue.
cc @DeFactoCepti since you mention selection and deselection times being slower, this video may be of interest to you. It seems something in the new selection logic may be interrupting the ability to drag the object. It legitimately takes me twice as long to get any (building) work done while this bug is present. If this bug is related to this feature, please consider reverting the update until the issue is fixed. 
A performance update is always welcome in my books. Thanks! This is great for games that use a ton of parts.
I am very grateful for this update, you guys have really been doing an awesome work in 2023. The performance got on my nerves at times, but it seems to be smooth now. Thank you very much, this makes studio more fun to use when working with large scale projects.
I don’t know how accurate this number is, but my Studio no longer crashes when I try to select everything. So I hope its correct. Good update.
This might be related to a change I made, will put it on my todo after the weekend.
It seems like ever since this came out, the selection tools in some places I’ve made are completely broken. I can’t figure out what’s causing this either since not every place does it. I’ve deleted every possible thing in the workspace too and that doesn’t fix anything either.
I’m not sure when this started, but I can no longer paste a decal/texture and apply it to anything. Now it just pastes into the workspace like a normal part.