Globe of Invulnerability - before or after?

[Specter]

So GoI says:
"Any spell of 5th level or lower cast from outside the barrier can’t affect creatures or objects within it, even if the spell is cast using a higher level spell slot. Such a spell can target creatures and objects within the barrier, but the spell has no effect on them. Similarly, the area within the barrier is excluded from the areas affected by such spells."

Is this intended to work only with spells cast after the barrier is in place, or those that have already been cast? For instance, would this barrier cancel an Insect Plague for those inside it? What about spell-affected people entering the barrier, such as a Slowed character?

[Tanarii]

How can a spell be cast from outside the barrier if there was no barrier when it was cast?

[Easy_Lee]

I'd say it cancels effects inside the barrier that originate (as in, the source of power or concentration maintaining that spell) from outside the barrier, but this is purely DM fiat.

[DarkKnightJin]

I'd rule that GoI prevents spells cast after it goes into effect from affecting targets inside it.
It's not Dispelling stuff. It's preventing new spells of 5th level or lower from taking effect.

So, a Hasted Fighter waltzing in to lay the smackdown on the trapped BBEG will remain Hasted upon crossing into the area.