The tbrsize heuristic in pfctl(8), used to set tbrsize when no size is given in the config file, results in poor regulation at high interface speeds (empirically somewhere north of a few Gbps on current equipment). This becomes a potential issue for systems post-r338209 (lifting of 32-bit bandwidth limit).
This adds a new larger value that is applied for interface speeds > 2.5 Gbps. 2.5 Gbps is the highest standard rate that could be used prior to r338209, so the default behavior for all existing systems should remain the same.
The value of 128 was chosen as a balance between giving the interface driver enough work to do per dequeue loop and keeping small the likelihood that a greedy driver will dequeue more than it can give to its hardware (and thus drop the remainder internally), across a reasonable range of average packet sizes (assuming a typical minimum hardware ring size of 1024 for 10Gbps+ interfaces).