Catalina 10.15.7 super slow after installation

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

This is not a problem that is intrinsic to 10.15.7, it relates to something in the individual configuration or installation of other softwares that are not compatible. I say this because where I work has over 3000 Macs being utilized and we recently had a mandatory update to 10.15.7 including all the additional supplemental and security updates. This is mandated by the US government, we are a contractor, and use of earlier MacOS is not permitted by that agency for IT Security reasons.

With these thousands of Macs in use now running 10.15.7 (Big Sur is not permitted yet, until some internal tools we use get updated for compatibility), on the internal message boards I have seen just a handful of problem reports, most of which have to do with password resets, none apparently like what you are reporting. So I think it is important to examine what has been installed on your Mac and figure out what might be causing the slowdown.

When you boot into Safe Mode, is the Mac still slow? Have you already reset NVRAM and SMC? How much free space do you have? How much memory? What type of internal disk do you have (SDD or HDD)?

One thing that may bring resolution to this, is for you to download and run Etrecheck, giving it full disk access. Paste the output here using the icon you see in the lower right of the "reply" window, an icon that looks like a paper page, for "Additional text." Once you do that, everyone reading this can inspect and from the collective knowledge of readers of this forum, something you have installed may be identified whose removal could speed up your Mac.

[Note added after the fact: I see that my post looks somewhat redundant with Old Toad's, who posted while I was typing! Old Toad is one of the people who may be able to find the culprit from your Etrecheck output that is slowing down your Mac]

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS. The Built-in Security of Big Sur is all that is required.