Speed is incremental. It does do something, despite what the other players say, but it's too marginal a benefit. The stat hint says that speed makes the miner better at fighting, but each speed upgrade is a very small fraction of a second. I could handle the leveling up for pretty much any other stat. The miner isn't designed to take too many hints; I'd say ignore the sword stat outright in the early going simply because mimics will be too weak to cause much of a problem. Get the pickaxe up and level up the mouse. But the speed is what hampers the game.
As an idle game, the whole point is to get to where you don't have to babysit it. You can leave it alone and see gains when you come back. With over 20 in pickaxe and sword and over 30 in speed, I left the game alone on Level 6 as a test. The game ran itself for four hours with no other programs or tabs to potentially slow it down, and I turned the rig down to allow it to run as fast as it could. The cat mined a whopping 200ish gold. By comparison, even at level one mouseclicks it'd have taken me a few seconds to accomplish what the game took 1/6th of a day to do. Given the amount of gold it takes (and consequently the amount of idle time or clicks it takes) to achieve a high enough speed that you can actually leave the game to run on its own, this isn't really an idle game. It takes tens, possibly hundreds of thousands of clicks to get the miner to the point that it can be left alone to accomplish anything significant, and I'm not killing myself to do that. The Skinner Box isn't that appealing.
Now, not to say you couldn't build a better game, there's the elements of a classic in this. Here's what I'd recommend.
1) Decrease the spawn rates of mimics, or give the cat something for fighting them. Your idle game features something that is solely designed to kill time and reduce the effectiveness of the resource gathering. Given that the mimics get stronger with how much gold and gems the character is carrying, this also makes it to where you can't leave the character idle for too long because then they'll be attracting final bosses left and right and never get anything done. Remember what the purpose of an idle game is. So either make mimics less of an issue, or give the cat experience or the possibility of item drops that could temporarily improve performance. Items wouldn't have to be anything major, your basic stat boosters would work. Healing, improving mining, improving fighter, improving speed... maybe even light the candle on the character's head to scare off mimics for a time; you could tie the buffs to an in-game timer rather easily. At least then whenever a mimic popped up it wouldn't only be viewed as a waste of time.
2) The games that take place in this shared world always have a knack for good backgrounds and visuals... I don't see why an idle game should be any different. The basic cave is pretty bland and just stays that way. No matter what the mineral mined, it still looks like the same cave. If we're bothering to go to different levels of a cave then maybe show us different cave visuals to at least break up the monotony. Especially as this game requires a lot more active participation from the player compared to a lot of other idle games, if we're staring at the screen that much something nice to stare at would be good.
3) Adjust the mineral stats for HP and rewards. It quickly becomes apparent that the rewards for mining do not equal the effort it takes. That is to say, I understand math and the math doesn't add up when descending to levels with tougher minerals. For instance, at Level 5, sandstone has 17 health. If I wanted to maximize returns on effort, then both the pickaxe and the mouse need to have at least 17 power in order to one-shot the sandstone. The return on that effort is eight to ten gold per click, or per action from Wiskers; averages out to nine gold per action. Now go down to the next level and the limestone has 22 health, greystone has 35 health, then so on 54, then 82, then 100. So to one-shot those minerals, I'd have to get the pickaxe or mouse power up to those same numbers, which in the game's system would take forever to accomplish both because speed is so marginal an upgrade and because the mouse clicks are upgraded with gems, that can only be gained one at a time. All this is to say that by the time somebody gets up to Peasant's Marble, sure they get 20 gold on average for cracking the ore, but cracking that ore would take them many more clicks or much more time from Wiskers to accomplish. That is to say that the player is effectively punished for descending in level, rather than ascending. The numbers under the hood need to be reworked so that it's not a long, slow grind to get to a new level, just to realize that you were better off at the level you were at before. You discourage advancement and can actually lock off the extra work you did from somebody seeing it because you incentivized them not to go and look at it.
4) Add in helpers. If speed can't be altered at least offer players the option to hire additional miners to assist Wiskers. You can even give them all their own stats to track; have different miners at different levels, make the metagame run wild if you like. This idle game actually has the advantage over other idle games that it doesn't need to run so many algorithms and resources that you end up slowing down some people's computers... this is still just basically one cat following a very short set of commands. So hiring additional miners that can share resources, who can split up so you have some weaker ones staying behind at the upper levels to mine slowly or who can stick together to effectively mine beyond their level by tackling the tougher stuff together, that would be neat. Especially if, like in your city-building games, there were random events or boons or curses to affect each additional miner depending on their own level or what level of the cave they were in. That would add a great deal of strategy to the game that most idle games just don't do.
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Don't think I'm saying this is a bad game, it's not. I understand it was made in a very short time. I'm reviewing what's here as if it were a standalone product, but with what you have here and given what we've seen from you before you could actually make a pretty revolutionary idle game without having to eliminate any of the features you've included in the game.