If you're talking about speed:
* Quickly
* Rapidly
* Swiftly
* Hastily
* Expeditiously
If you're talking about gradualness:
* Suddenly
* Abruptly
* Immediately
* Instantly
If you're talking about a lack of enthusiasm:
* Eagerly
* Enthusiastically
* Passionately
* Vigorously
So, you need to consider the specific meaning of "slowly" in your context to find the most appropriate opposite.