stamp_par no speed-up on windows

[maschere]

I was checking performance and noticed that stamp_par works fine under Ubuntu, but on Windows 10 (tested on an Intel and an AMD machine) stamp_par is only as fast as stamp, no matter how many workers are used. Used R 4.0.2 on all three machines.

set.seed(2018)
data <- cumsum(sample(c(-1, 1), 40000, TRUE))
tsmp::stamp(data, window_size = 1000, verbose=2) #takes 2 minutes
tsmp::stamp_par(data, window_size = 1000, verbose=2, n_workers = 2) #no speed-up, 2 minutes (under Windows)
tsmp::stamp_par(data, window_size = 1000, verbose=2, n_workers = 8) #still no speed-up, 2 minutes  (under Windows)