Would an unnecessarily long password degrade performance of a WPA2 wireless network?

No. It doesn't matter how long your passwords are because that value is never transmitted during the WPA-PSK Key Exchange. Instead a CMAC is calculated based on the secret key, the client and server id, as well as client and server provided large random values.

Regardless of how large your password is the resulting CMAC for WPA2-PSK will always be 128 bits:

MAC_P = CMAC-AES-128(AK, ID_P||ID_S||RAND_S||RAND_P)

No. A long passphrase will not cause slowdown.

The passphrase you type in is first converted to a fixed-size cryptographic key (e.g., a 128-bit key); no matter how long the passphrase, the cryptographic key will be the same size. Encryption and decryption take exactly the same amount of time, regardless of the length of your passphrase.