so if I look at each part of my code. the first 4 lines will take 20 ms

input_data = input_data.replace('\r', '').replace('\n', '')
part2_data = [[i//2 for _ in range(int(x))] if i%2==0 else ['.' for _ in range(int(x))] for i,x in enumerate(input_data)]
part2_data = [ x for x in part2_data if x!= [] ]
part1_data = [y for x in part2_data for y in x]

The part1 for loop will take 10 ms.

The for loop to set up next_empty_slot_by_length will take another 10 ms.

The part2 for loop will take 10 ms, too!

and adding up the part2 checksums will add another 10 ms.

So, in total, it will do it in ~60 ms, but python startup overhead seems to add 20-40 ms depending if you are on Linux(20 ms) or Windows(40 ms). both are Host, not virtual. Linux usually has faster startup time.

I am not sure where I would see a speed up. It seems that the startup overhead makes this just slower than the other top performing solutions which are also hitting a limit of 40-60 ms.