Re: Less performance oppening connection from 5.x to 8.x driver

[Mike Frank]

Very good that you are upgrading from a security standpoint.

It's likely your performance is due to use of modern strong cryptography in 8.0.
At the end of this I explain an option you can look into.

SSL is on by default in 8.0. Highly recommend to leave it on.
8.0 Hashing is 5000-10000 rounds of SHA2 hashing. This is standard best practice.
Past initial connection this cost goes down.

So - if you are running without SSL with 5.1 (not recommended)
And you are running with the easy to break hashing - SHA1 or MD5 running one cycle.
Then there you go.

Which version of OpenSSL can matter. If you are on linux - openssl 1.0.2 is significantly slower than 1.1.1 and 3.0. MySQL 8.0 dynamically links to the optimized OpenSSL on the OS.
EL7 - OpenSSL 1.0.2, OpenSSL - EL8 1.1.1, OpenSSL - EL9 3.0. Windows is 1.1.1. Ubuntu, SuSE - like EL - whatever is on the OS.

MySQL 8 also supports FIPS mode etc.

So if you are on a OpenSSL 1.0.2 - you will get a significant performance boost by moving to an OS with either OpenSSL 1.1.1 or 3.0 versions.

Hope this guidance helps. Stay secure.