DMD release build being faster than debug!

[Per Nordlöw]

I just noticed that building DMD~master via

     make -f posix.mak BUILD=debug

currently takes 3.2 secs while building it via

     make -f posix.mak BUILD=release

takes only 3.0 secs on my Ubuntu 18.04 64-bit machine!

Are there more DMD switches other than `BUILD=release` I need to 
activate to produce the fastest possible compiler binary? Apart 
for compiling it with LDC, that is.