should I go to Big Sur

Unless you are using a 2013 or later MacBook Air, then you cannot upgrade to Big Sur anyway. If you have 4GB of RAM in the MacBook Air, I would strongly advise against an upgrade to Big Sur. Not enough RAM, and it would be worse than slow.

There is considerable preparatory work involved before upgrading to Big Sur:

1. You will need to free up 35.5 GB of available storage before you can commence the upgrade
2. Big Sur cannot run 32-bit applications, or support printers/scanners with 32-bit drivers, or 32-bit vendor software.
   1. Check with the printer/scanner vendors to see if they have released drivers since Nov 2020. If they haven't, or have abandoned your device, then you may have a paperweight, and a new device expense.
   2. Run the free Go64 application to list the 32-bit applications that you have installed on High Sierra.
      1. You will need to check with each third-party vendor to verify that they have 64-bit applications available that are compatible with Big Sur. Some vendors may charge upgrade fees for their 64-bit versions.

The reason to upgrade to Big Sur would be current Safari browser, and operating system security improvements and an Apple supported operating systen for another two years of security updates. Additionally, third-party vendors may abandon new product updates on macOS High Sierra, as it is not supported by Apple now.

Depends, does all you software work in 10.15 & Big Sur?

Caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/