Short answer: none.
A router in such a network would be using TCAM hardware (ternary content-addressable memory).
Normal RAM is used by addressing a cell (bit/byte/word) and then reading or writing its content. Searching for a specific content requires you to read memory cells sequentially and comparing each of them to the sought value.
Content-addressable memory features a kind of reversed access method where you specify a value and the CAM returns the address for the first cell matching that value. This is widely used for e.g. destination MAC lookup in layer-2 switching.
Ternary CAM additionally supports a mask value for each memory cell, allowing for partial matches. This allows for expanding the use cases to layer-3 prefix lookup in routing tables (or NAT tables, firewall rules, ...).
Accordingly, a route lookup using TCAM is a single step, regardless of the length of the routing table (the maximum table length is defined by the TCAM size). Thus, there's no difference in processing delay due to the position of the routing entry in the table.
Even with software-based routing you would probably find a compare-first-64-bits-and-second-64-bits-only-when-matching algorithm which is only a tiny bit faster when the second matching can be skipped, so you wouldn't ever be two times faster for a 64-bit prefix.
That said, you most likely wouldn't find a /127 prefix in a "big core" router in any case...