after installing BigSur starting up very slow and all programs as well. why

Immediately after installing a new OS, it is building caches, indexing the drives, and other things, so it may be initially slow to respond. However, it should not last long.

If it remains that way, you may have some software installed that is not compatible with Big Sur and is causing the slowness you see. As mentioned already, Etrecheck may be able to highlight the problem programs.

Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. The Report will Not Reveal Any Personal Information. Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.