slow startup

Paul4You wrote:

After installing Big Sur my iMac is slow during startup. Anybody suggestions?

iMac Retina 4K 21,5 inch 2019 3 Ghz 6-Core Intel Core i5, 16 Gb 2667 Mhz DDR4

Did you try a Safe boot, to clear caches files, you do not even have to log in here, reboot as normal and test.

SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc.

Also, have you installed or run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

If you have then 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.

Copy the report

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

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.

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