When I press the play button, it loads slower than usual then once loaded a crash screen for MC appears.

This isn't nearly enough information. For instance, where is the crash log?

Incomplete without crash report ([minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/crash-reports/crash-<DATE>-client.txt). Please attach it so that we can diagnose your issue.

Where exactly will I find this at?

As stated: ([minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/crash-reports/crash-<DATE>-client.txt)