MacBook Air 2018 login slow after altering home folder name

It sounds like you didn't perform the procedure correctly as outlined in the following Apple article.....every step must be performed correctly (there is only one optional step):

Change the name of your macOS user account and home folder - Apple Support

Did you create or have a second macOS "admin" user account?

If you do have another macOS admin user account, then you can log into that user account & use the information in the Apple article I linked to correct the mistake(s).

If not, then I hope you made a good backup prior to making the catastrophic change since I don't believe there is any way to correct the mistake without having a second macOS admin user account. You will need to erase the computer and restore from a backup made prior to the mistake. If you have no backup, then maybe you have some of your data on external media or even in the cloud.

Here is an Apple article with instructions for erasing the computer:

Erase your Mac and reset it to factory settings - Apple Support

If you have no backups or some of your important files are not stored elsewhere or in the cloud, then I guess you can try using Target Disk Mode in order to attempt to connect the "broken" Mac to another Mac assuming you can properly authenticate with the T2 security chip. I have no idea if this is possible with your broken configuration. Hopefully you won't encounter the T2 authentication bug which I've seen on some systems. This is your only option to recover data without a backup or having copies stored elsewhere.

FWIW, changing the (short) username, or the home folder name, has always been a dangerous endeavour. I would always recommend against doing it.