The .NET framework has nothing to do with the base operating system, but is used by many lumps of software to obtain services from it.
The "Preview" is not a release version; it will be replaced by the proper one in today's batch of fixes & updates - the second Tuesday of each month is known as "Patch Tuesday" (Microsoft is not the only company which uses this schedule).
I don't know why the .NET team push out Preview versions, but most in the IT industry avoid installing them.
Slowness is usually caused by software which you don't need hogging resources which you do. Going through the list under "Settings", "Apps" and uninstalling things you don't want is a good thing to do. Then check which programs are being run at startup and disable the ones you think shouldn't always be running. You can reach the list in Task Manager; it is one of the tabs which appears once you select "More Details".
Blurred pictures though? Do you mean image files stored on your PC, or those being brought down by your browser to view on screen?
If the former, then you may be using the wrong image viewer program - Windows comes with more than one.
If it is things such as Ancestry images, there are many possible reasons, but the top three are:
1. Ancestry being overloaded; they need to buy more bandwidth from Amazon (it's all in Amazon's cloud service).
2. The speed of your link to the internet. If you can download other things quickly, it's not this.
3. Using the wrong browser. The old version of Microsoft Edge was poor at this, and failed to display quite a few images from FindMyPast. The new version, based on Chromium (as is Google's Chrome browser) is much better. Try pressing F5, or clicking arrow-going-in-a-circle icon to refresh the page. With the bits from the first attempt already in the cache, it may be more successful for the second attempt.
After that we get into speculation.
Personally I doubt that your Disk Cleaner will make much difference. Their writers make claims about things such as a cluttered Registry. Removing registry entries is a bit like removing something from the far corner of your attic and expecting that to speed up your trip from the front door to the kitchen. They won't even reduce the size of the Registry files once entries are marked as Deleted, because those files are continuously in use by Windows, and protected from modification except by the official system calls.
The included "Disk Clean-up" (under Windows Administrative Tools) does a decent job of tidying up the main disk space hogs, especially if you use the "Clean up system files" button. It doesn't empty the browser cache for all browsers though, so you may wish to do that from within your browser's menu.