Not only does the credit bureau max out their password length, you have a small list of available non-alphanumeric characters you can use, and no spaces. Also you cannot used a plused email address, and it had an issue with my self hosted email alias, forcing me to use my gmail address.
Both Experian and transunion had no password length limitations, nor did they require my username be my email address.
Update: I have been unable to log into my account for the last 3 days now. Every time I try I get a page saying to call customer service. After a total of 2 hours on hold I finally found the issue, you cannot connect to Equifax using a VPN. In addition there is no option for 2FA (not even email or sms) and they will hang up on you if you push the issue of their security being lax. Their reasoning for lax security and no vpn usage is "well all of our other customers are okay with this".
A 20 character password of case insensitive letters and numbers is quite unbreakable (taking billions of years to brute force). Still, what a strange way to announce your database is old and you probably aren't hashing your password with anything stronger than MD5. Or worse.
A hash has a fixed length, including MD5. There's no reason to cap password (input) Iength. You can hash the whole bible and still get the same length hash. So either they don't even hash it, they're idiots, or they try to be unnecessarily cautious to avoid some other limit / overflow, like POST max size (which would still be counted in at least KB, not several characters). The limit on what special characters you can use is also highly suspicious - that's not how you deal with injections / escaping your inputs.
Hashing takes longer the longer the string is, so it technically could impact performance if many people with very long passwords log in simultaneously. 20 characters is ridiculous though, you could probably cap it at hundreds and still be completely fine.