All in F# going to try and keep all solutions immutable, but going to try and get every solution under 1 second and will post runtimes, and wether i had to make them immutable.
All will be timed by (depending on wether i'm on my windows or linux machine at the time) time dotnet [output] or Measure-Command { dotnet [output] }. I'll run it three times and post the range.
day1: 204-211 ms
day2: 128-132 ms
day3: 138-143 ms
day4: 147-150 ms
day5: 147-154 ms
day6: 137-146 ms
day7: 212-225 ms
day8: 359-369 ms
day9: 168-190 ms
day10: 133-139 ms
day11: 424-439 ms
day12: 123-129 ms
day13: 158-159 ms
day14: 205-206 ms <-- used mutation (same code swapping dictionary with an immutable map took ~3 minutes)
day15: 231-244 ms <-- used mutation (version with same logic, but using a Map took ~86 seconds)
day16: 183-211 ms
day17: 438-445 ms
day 18: 205-220 ms