It applies to the cast delay as it is cast. Double or triple casts really make a difference here, since the chainsaw negates all cast delay, including the one added by any other spells. Combined with a low recharge wand you can easily build a machine gun that fires a projectile every frame. That easily clears up to level 3, but beyond that, you'll want something more sophisticated.

For that, Triggers are your best friend. Chainsaws are next to useless as a weapon, right? Put one (or five) behind a Spark Bolt with Trigger (and yes, you can and should trigger multicasts), your sparkbolt goes from 3 damage to 15 (or 65...). You can also add modifiers behind a trigger. Remember, modifiers affect anything that's cast at the same time, just like the chainsaw hidden effect. So if you take a setup like

• Spark w/ Trigger
• Damage Plus
• Triple Cast
• Chainsaw x3

Suddenly every shot does 75+ damage. Swap out the Damage Plus for a Bloodlust, you're all the way up to 140+. That will oneshot most of what you'll encounter until level 5

All of that is doable before you enter level 3, if you're lucky, and this is still a fairly simple example. Make good use of modifiers, many things can stack to absurd amounts. Another favorite is Piercing Shot + Trigger, because that triggers the secondary effects without destroying the original projectile. Add in homing and you've effectively built a very angry bee doing 140 damage every frame. Or launch a nuke, if that's what you put in as a payload.

I wouldn't recommend getting distracted by other areas until you're at least somewhat consistently getting to the jungle. After that, try to defeat the alchemist (left from Collapsed Mine, past the darkness). You will die, but after you eventually do it you'll unlock some truly amazing spells.

As for digging, a fairly safe and consistent option that is common in early areas is Luminous drill. It can break everything, but is fairly slow. Once you find Add Mana and 2/3x Scatter Spell, it becomes amazingly fast.