This is the first time somebody reports performance problems after patching the assemblies and I can't tell what causes it until debugging the app.
There are a couple of steps you can try to troubleshoot this issue and find out whether Telerik is at fault, if so, which control.
First, try to remove Telerik controls from the page. If the Site still works slowly, that must be a site configuration problem. However, If the site speeds up, that indicates that Telerik Controls have something to do with the performance and will require attention. Assemblies on their own do no harm if referenced in the project, and the Telerik Controls should be okay unless there is a custom code that makes a time-consuming job, hence decreasing performance.
If there are multiple Telerik Controls on the page, you can also try removing them one by one. That would help to find out which of the controls is at fault.
We can investigate the problem, but for that, we would need a sample project that we can run locally or if you could describe the steps that will help us replicate the issue, that would help as well.
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