https://docs.paloaltonetworks.com/pan-os/11-0/pan-os-admin/virtual-systems/customize-service-routes-for-a-virtual-system/customize-service-routes-to-services-for-virtual-systems

When sizing a decryption firewall deployment, two factors that should be considered are the encryption algorithm and the TLS protocol version. These factors affect the amount of resources and processing power that the firewall needs to decrypt and inspect SSL/TLS traffic.
The encryption algorithm is the method that the server and the client use to encrypt and decrypt the data exchanged in an SSL/TLS session. Different encryption algorithms have different levels of security and performance. For example, AES is a symmetric encryption algorithm that is faster and more efficient than RSA, which is an asymmetric encryption algorithm. However, RSA is more secure than AES because it uses public and private keys to encrypt and decrypt data, while AES uses a single shared key. The firewall must support the encryption algorithms that are used by the servers and clients that it decrypts, and it must have enough CPU and memory resources to handle the decryption workload12.
The TLS protocol version is the standard that defines how the server and the client establish and maintain an SSL/TLS session. Different TLS protocol versions have different features and requirements for encryption algorithms, cipher suites, certificates, handshake messages, etc. For example, TLS 1.3 is the latest and most secure version of TLS, which supports only strong encryption algorithms and cipher suites, such as AES-GCM and ChaCha20-Poly1305, and requires elliptic curve certificates. The firewall must support the TLS protocol versions that are used by the servers and clients that it decrypts, and it must have enough hardware acceleration resources to handle the decryption speed34.
The number of security zones in decryption policies and the number of blocked sessions are not relevant factors for sizing a decryption firewall deployment. The number of security zones in decryption policies only affects how the firewall matches traffic to decryption rules based on source and destination zones, but it does not affect the decryption performance or resource consumption. The number of blocked sessions only indicates how many sessions are denied by the firewall based on security policy or decryption policy rules, but it does not affect the decryption capacity or throughput56.

https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClEiCAK

The cause of the unsecured website warnings is that the forward trust certificate has not been signed by the self-signed root CA certificate. The forward trust certificate is used by the firewall to generate a copy of the server certificate for outbound SSL decryption (SSL Forward Proxy). The firewall signs the copy with the forward trust certificate and presents it to the client. The client then verifies the signature using the public key of the CA that issued the forward trust certificate. If the client does not trust the CA, it will display a warning message. Therefore, the forward trust certificate must be signed by a CA that is trusted by the client. In this case, the administrator has installed the self-signed root CA certificate in all client systems, so this CA should be used to sign the forward trust certificate. However, as shown in the screenshot, the forward trust certificate has a different issuer than the self-signed root CA certificate, which means it has not been signed by it. This causes the client to reject the signature and show a warning message. To fix this issue, the administrator should generate a new forward trust certificate and sign it with the self-signed root CA certificate12. Reference: Keys and Certificates for Decryption Policies, How to Configure SSL Decryption