Previous answer is wrong in-part. Yes there is no direct correlation between Meltdown and UEFI attacks (yet) being seen in practice or in theory. But based on the fact that meltdown allows you to read kernel protected memory, the possibility to read any data in memory by incorrectly training the branch predictions.
Secure boot, UEFI and BIOS all rely on total integrity and confidentiality of data within the computational environment. More importantly these vulnerabilities totally subvert Trusted-Execution Environments which are supposed to be cryptographically secure processing units logically separated from the CPU. But speculative execution vulnerabilities allow you to bypass that encryption.
It's only a matter of time before some APT group produces the next Strontium UEFI bootkit and leverages this vulnerability.