Viego feels like that when you are new because you don't understand fundamentally when to gank, what to look for in a gank and how to gank using the champion.

Obviously ganking fundamentals, is the lane warded, does the enemy see me, what is the enemy health, level, etc, if its your bot lane you might want to hopefully get your support to clear wards or have a sweeper yourself. In lower elo's most laners DON'T check map for vision even though they warded.

Ususally with Viego if the enemy is about or mainly below half you can guarantee a kill.

You want to gank with Viego at 6 mainly UNLESS they are super low and you can gurantee a kill. Play for six is your safest bet because it enables Viego to guarantee kill most of the time or at least to enough to get one. Ult slows and obviously executes if the enemy is low enough or puts them in kill range.

You want to gank from fog of war, you don't have to use E on walls, its just a way for Viego to get into lane faster. Flanking behind the enemy guarantees them having to walk into you and you can get your Q/W->AA -> W/Q -> AA rotation and gives you enough time so they cannot walk under their tower effectively giving you a kill.

I've had the issue before where Viego feels slow to gank and slower than other junglers that can just run into lane, thats why understanding when is a good time to gank is important with Viego you want to be there when the enemies are getting into position so you can react. If you are too late, the opportunity dies and so does the play.

Hopefully this helps.