Time machine backup takes very long

vsrvnn wrote:

after upgrade to Big Sur. my time machine drive failed. I replaced with new drive and now it takes whole day for backup. Each time preparing back also takes hours.

You can see your formatting of the external TM backup drive from Disk Utility.app

If a Time Machine backup takes longer than you expect ...

If you want to use a new external drive as a TimeMachine backup drive with Big Sur, format it as APFS. It will probably be faster.

Two things to think about:

• use the TM backup disk only for TM backups
• if you reformat: it will first be totally erased

Note: can't you repair your old backup drive with DiskUtility First Aid?