Xcom 2 Crashes During Mission
Found what was the problem and a selution to this for me, hope it is the same problem you are having. As you have noticed it takes longer and longer to get up load/save files as you save more and more.
Xcom 2 Crashes During Mission 4
It seems there is a problem with the save files using up resurces on pc's witch result in games crasing when you have to many save files. I have had this problem sevral times, and some others sayd lowering the reselution helps in option ( since the pc uses less resources then ). But i found a better selution to this problem.
Xcom 2 Crash Log
Go to 'USERDocumentsMy GamesXCOM - Enemy UnknownXComGameSaveData' on your pc ( or the other folder at end if playing war of the chosen ). Then delete all files exept the first ( config file ) and the 3-4 last once ( your last saves ). Use shift click to mark all those at once and delete. Make sure not to delete the first and last file. Now start the game and save/load should go mutch faster in the game, and for me it made the crash at mission start go away.