Using the standard encryption that Fusion 12 provides for virtual machines required to obtain the TPM module does impact performance. Every read from the virtual disk must be decrypted, and every write must be encrypted.  It's hard to say exactly how bad that is in practice, but that's additional CPU resource that's required. 

Knowing what I know now, I would never recommend someone use the experimental virtual TPM of Fusion 12. There are too many restrictions on it - the least of which being that you can not move that VM to another machine because there's no easy way to get the encryption key required to open the VM.

You might want to look at the Fusion 22H2 Tech Preview if running Windows 11 VMs - even on Intel Macs. The encryption options in the Tech Preview allow a "partial" encryption of the VM (only files outside of those that make up the virtual disk). Similar to what the experimental vTPM feature did, the Tech Preview's partial encryption allows configuration of the TPM device without the overhead of encryption/decryption of the virtual disk.  Unlike the experimental Fusion 12 vTPM, it provides much better management of the encryption key. It fixes the deficiencies present in Fusion 12's experimental vTPM that made it unworkable in practice.

The encryption/TPM features present in the Tech Preview are anticipated to be present in the next Fusion version.