Why am I training so slow?

You will only train at a reasonable pace if you consistently use your skills. Never use a "regular" attack for any class after level 25, and if you're an assassin or mage you will probably need to use skills WAY before that. To keep up the pace of your mana-consumption, you must buy potions to consistently up your mana. By level 35 mages should hold AT LEAST 60 mana potions so they don't have to go back and forth to buy more.

Additionally, players must always have HP potions ready. Sitting still for 10 minutes every time when hit is not a fast way to train. By level 35, any class should have 50 ORANGE potions.

Upon reaching level 40, start scrolling weapons, on top of using your skills, to keep a reasonable pace of experience. 60% scrolls are the general recommendation, and if feeling lucky, buy 10% scrolls off of other people. 100% are a safe choice, but add less than 60%.

For assassins, get the best stars you can afford at all times. Kumbi tend to be the ones people get the most. Or, if funded, get icicles at early levels and kill much faster. If you have MapleStory iTCG, and play in MapleGlobal, AND are funded, you may want to try for Crystal Ilbi, which is +29attk.

Actually, getting a +7 100% claw is more important than getting stars, especially with Meba. Also, once in possession of Kumbis/Icicles, start spending money on work gloves scrolled with attack. Get 6 attack gloves before getting Tobi, 8 attack gloves before getting Steely, and 10 attack gloves before getting Ilbi. Also, starting from level 50, consider getting +5 or more 60% scrolled claws. Just look at the next upgrade cost for your claw, glove, star, and determine which one is cheaper. Don't go out thinking that getting a better star is the only option to raise your attack.

If you are low on money, here is the list of priorities if you want to continue training at a fast pace:

HP potions and Mana potions
Most powerful Weapon/stars money can buy
Scrolls for weapon Attack, or intelligence/magic Attack if a Mage
Best Top/Bottom/Overalls money can buy
For archers, starting with level 50, overalls are the main equipment. Overalls feature 10 slots, which can be scrolled easily for dex. Buy 100% dex scrolls from the various NPCs for 35,000 and get an easy 10 dex. This is also an option at level 30 with the sauna robe, or level 20 with the bath towel if your version has Showa town, but generally players at that low of a level don't have enough money to afford it. It is recommended that you don't waste money on gloves or shoes, unless they are upgraded with weapon attack, speed, or jump. The level 15 or 20 glove will suffice for a while for any class; in fact, most mid to higher levels (level 50+) use a level 10 work glove (upgraded with weapon Attack)!

So what does this list mean? It means, if poor, buy as many Health and Mana potions as you can. If you saved up enough money, then buy a more powerful weapon. If you still got money, then scroll your weapon. If you still got money, then buy your armor.

Parties are another good option, as mesos from other members' kills will go to everyone.
http://strategywiki.org/wiki/MapleStory/FAQs