Fastest pre-HM mining speed is rather dependent on the material being mined.
For dirt, sand, mud, clay, silt, snow, etc. the fastest would be a light bone pickaxe + mining potion + mining outfit.
Bone pickaxe has a mining speed of 11. With a light prefix, this bumps down to 9.
With mining potion and mining outfit providing a hefty 55% boost in mining speed, this bumps down to 4 mining speed, or 15 blocks per second.
For hard materials like stone, bricks, ores, etc., the fastest would be a light reaver shark + mining potion + mining outfit.
Reaver shark has a mining speed of 18. With a light prefix, this bumps down to 15.
With mining potion and mining outfit providing a hefty 55% boost in mining speed, this bumps down to 6.75 mining speed, or 6 [always rounds down, unless it is halfway in between two integers. In this case, it would be the even number of the two integers]. That is equal to 10 blocks per second.
Melee speed bonuses from armor and accessories have no effect on mining speed. Only the tool's melee speed bonus has an effect on mining speed.
Just an FYI: There is no melee speed bonus for a tool. There is only a speed bonus.
The difference between the two is that the former affects useAnimation, while the latter affects the useTime.
I mean, when you look at the light prefix, it says "+X% speed", not "+X% melee speed" like the accessories do. It can be confusing, but yes, they are very different.
That's pretty much the reason why tool/weapon prefixes affect mining speed. Melee speed has nothing to do with mining speed at all. In fact, why do I even use them in the same sentence?
EDIT: Regarding bombs...
Pulling this out of my knowledgeable
, I'd estimate the rate in which bombs destroy blocks would be around 4-6 per bomb [to make a 2-wide hellevator], assuming you're racing against a bone pick + mining outfit/potion guy to the underworld [if you're racing against clearing an area up, congrats, you win]. With a useTime of 24, or at a rate of 2.5 bombs per second, you can achieve a speed of around 10-15 blocks per second, or an average of 12.5 blocks per second.
Whereas the bone pickaxe would mine at a rate of 15 blocks per second max on soft blocks, and 7.5 blocks per second on hard blocks. Given the majority of the cavern layer is made of stone, and the underworld and underground/surface is made mostly of soft block, let's assume that digging straight to hell is 75% hard blocks and 25% soft blocks. That would mean that your average would be around 9.375 blocks per second.
The reaver shark would mine at a rate of 10 blocks per second regardless.
I would hypothesize that bombs would still outmine both pickaxes in hellevator-making, making bombs the fastest way of mining in pre-HM in both area-clearing and hellevator-making.
Or is it?
Dynamite? Maybe. I don't have enough knowledge in my
to pull off to provide even a bit of info to compare.