Assigning to rantonysamy@ for confirmation.
Does the description above describe your situation exactly?
(I assumed that "zip unblocker" in the orignal report was typo of "ZIP archiver.")

Also, I'd also like to ask you sample and upload 2 or 3 seconds of record by chrome://tracing with all record categories while the slowness is happening.

Please take a look at this bug which is also trying to fix the same issue :  812123  

At this moment we cannot conclude 812123 is the same issue or not. So it'd be helpful if we can get more exact situation description for this case.

812123 is the parent issue from which I split this issue, because the reported symptoms turned out different after the first report.

Here are the differences observed so far:
812123: Having mounted volumes that cannot read nor unmounted correctly. Task manager shows "Zip Archiver". Runtime error was confirmed in the console. Happened after opening a zip file.
813780: Without opening files app, the system performance is slow. Happened after powerwash evenly. Task manager shows ZIP unpacker.

Yes its exactly the same I experienced. 

I tried with a Panther, ASUS Chromebox CN60.
After recovery by the recovery USB drive, it was M63.
I added an account and then switched its channel to beta. Then it got this version:

Google Chrome	65.0.3325.89 (Official Build) beta (64-bit)
Revision	0
Platform	10323.39.0 (Official Build) beta-channel panther
Firmware Version	Google_Panther.4920.24.26

After update and reboot, I did not see the slowness of the system performance.
I also tried powerwash after it, but did not see UI slowness neither.

I found ZIP unpacker extension staying in the task manager, as well as Native Client module of the same app, but only in the first session after adding user account.
I did not experience UI slowness while that task exists neither.

It'd be helpful if we could see if the recent fix for 812123 have affected this, on the device and account which had this issue in the past.

This bug is marked as blocking 65 stable, which is going to try to promote next week, any chance we can get a fix this week?

This issue has not been confirmed by any other places than the original report, and haven't got enough information to specify the range of the problem. So we cannot take further action on this at this moment.

Having said that, we have fixed some bugs that might have been affecting this (#12). So we would like to know if it changed the situation in the original report by rantonysamy@.

Removing releaseblocker label by the reasons noted in #15. Feel free to update if this is still happening.
This is considered happening with specific status of a user profile, although not affecting all users. So it should be investigated further as a longer term plan.

testing this in beta qual build (M65) in the same device and ZIP unpacker still loads in the background along with Native client module ZIP Unpacker, for a fresh (power washed) installation. Does not really affect the performance of the system in any way.

Feel free to close it If the original bug is fixed and keeping the ZIP process is not going to affect the performance in any way.. 

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Thanks for clarification.
Closing as WontFix because we could see that the process runs after powerwash but didn't see it giving bad effect to the system performance, at least with the latest code.