[JASJAR, XDA Exec, MDA Pro ROM Development] dwallersv:
Not so sure this is a "don't try this" solution. If done properly, there should be no risk at all.

"Properly" looks like this: From your favorite registry editor (mine3 is FDC Task Manager), follow these steps:

Perform a full backup of your configuration, using something like SPB Backup or similar, that can fully restore your phone configuration from a hard reset, if something goes wrong

Open your File Explorer application (I like Resco Explorer), and minimize it into the background. You will need this to do the import if your registry editor doesn't have an import function.

Export entire tree from key HKCR\CLSID to a file

Delete key HKCR\CLSID

Using the task switcher, bring your File Explorer to the foreground. Import .reg file exported in step 1 by executing it

Check that import was successful by looking in the CLSID tree a bit

Reboot (soft-reset) via software reset function, or wait a few minutes and reboot via recessed reset button on device

Under no circumstances should you reboot the device in the middle of this procedure or suspend the device! Also, the registry is cached, so as a best practice, you should always use a software method to reboot (soft reset) the device, as the WM call to reboot flushes the caches. Hitting the recessed reset button does not, which is where this advice always comes from to wait a few minutes before resetting after registry changes. Most registry editors that I've seen (FDC Task Manager, certainly) have a reboot feature built-in somewhere -- use that.

Why does this work? I don't know with any certainty, but I have a theory. The registry is not stored as a simple ascii or unicode text structure. rather, the hierarchical structure is interpreted by Windows, and I believe hashed (or equivalent) to make searching and access faster.

Devices are shipped with a pre-loaded registry, with all the hashing/indexing information as-is from the development process of the manufacturer's implementation of WM. Over time, with lots of registry changes (the thing is being written to all the time), hashing/indexing becomes less and less efficient, and needs to be cleaned up. Registry optimizers regenerate some or all of this hash/index information to reflect the current structure of the registry, so that hashes "hit" correctly on the first search, or within a few retries.

So, I'm guessing that the hash tables for the CLSID tree are out of date, making searches longer, slowing down read accesses to the registry, and a lot in some cases if the hashing is really off. This may be the case for the SIP section, for some reason related to the development process that unfolded either at MS or the device manufacturer.

Deleting the key deletes all the hash tables for it too. Re-importing it recreates them fresh, and accurately. Hence, searching that part of the registry becomes blazing fast.

This theory would also explain the increase in speed one person reported after exporting, basically, the entire registry, then reimporting it.

BE MINDFUL THAT THIS IS PURE SPECULATION ON MY PART, INFORMED BY YEARS OF EXPERIENCE AND ACCUMULATING KNOWLEGE IN A CAREER IN SOFTWARE ENGINEERING. I MAKE NO CLAIM AS TO THE ACCURACY OF THESE IDEAS AS REPRESENTATIVE OF THE ACTUAL MECHANISM.

Just seemed to make sense to me.