One of the effects of the slow spell is particularly nasty for spellcasting targets:
If the creature casts a spell with a time of 1 action, roll a d20. On an 11 or higher, the spell can't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.
The counterspell spell says:
You attempt to interrupt a creature that you can see casting a spell.
Let's say Bob the Evil Wizard gets slowed, then tries to cast a spell but rolls an 11+ on the die, and is therefore only able to complete his spell on his next turn. In the meanwhile, it becomes Tom the Nice Wizard's turn.
Can Tom counterspell Bob on his own turn in that situation, considering that Bob is still in the process of casting his spell?