Hmm. Thinking a bit more about it, it might be hard to beat the FFT with when your signals are *that* long.
FFT is O(N*logN), while time domain is O(L*logN) where L is the number of lags. log2(60*48000) is less than 22(!) so the FFT will be *very* beneficial. Here you can have a look at a discussion of conv speed in Julia: https://github.com/JuliaLang/julia/issues/13996 , including a heuristic for choosing between FFT and time domain processing.
