I don't know how fuel metering in Wasmtime works and what its overhead is but keep in mind that Wasmi is an interpreter based Wasm runtime whereas Wasmtime generates machine code (JIT).

In past experiments I remember that fuel metering adds roughly 5-10% overhead to Wasmi executions. The trick is to not bump or decrease a counter for every single executed instruction but instead to group instructions together in so-called basic blocks and bump a counter for the whole group of instructions.

This is also the approach that is implemented by certain Wasm tools to add fuel metering to an existing Wasm binary.