Mining in Alien Worlds requires actual Proof of Work, a blockchain term to describe that a transaction, in this case an action of work, took place. To prevent network abuse and disincentivize automation, successful mining attempts must supply proof of work. When you mine in Alien Worlds, your device is trying to find a hash which is like a missing piece of a puzzle that is required to complete mining.
The tools and land NFT game cards have Proof of Work reduction statistics, represented by the lower left number in the pyramid icon. The higher the number, the less proof of work required to complete a mining action and the faster you can find the hashes and therefore mining will complete faster.
An analogy might be mining through dirt, pebbles or rocks, whereas it takes less time to mine through dirt or pebbles than rocks, and longer time if solid rock. Dirt would have a higher number than pebbles, which would have a higher number than rocks. The lowest number might represent solid rock and take the longest.
Note: PCs are faster than mobile devices such as tablets and phones and will therefore likely complete proof of work faster. Delays associated with a lower proof of work number will not be as noticeable on PCs as on mobile devices.