Well it's best to ignore the initial stat gain for things like carbos.
At lvl 100, Pokemon stats are decided by a combination of nautre, Individual Values, and Effort Values (EVs). EVs are the only things that can really be influenced by the player.
Each vitamin/mineral given to a Pokemon will increase its EVs in that stat by 10, up to a limit of 100. The maximum total EVs allowed by the game for all the stats (including HP) is 510, and the maximum possible for each individual stat is 252. Certain Pokemon will increase the EVs in specific stats - e.g. defeating a Raichu will increase the speed EV of the Pokemon by 3. Bit too complicated really, best to look on Google etc. for a proper explanation.
Higher EVs usually mean better stats at level 100, so if you want a Pokemon with a high speed stat, use Carbos 10 times until it says it won't have an effect and then fight Fearows and Raticates. Just an example...
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Carbos raises your pokemon's base speed.
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There are four. Timid, Hasty, Jolly and Naive.
Carbos for EV stats as well as the hold item Power Anklet Quick claw, may let your pokemon attack first and choice scarf will also increase your speed but limit you to only one attack.
Snowshoes are not an item available in any Pokemon game (That i know of) When you are in snow, the speed at which you walk is always kept that slow, you cannot change it, it is just the game.