Minecraft – Blaze Melee Attacks

There are quite a few tutorials for that, and these two designs seemed most notable to me.

First design (by Etho) is complicated, expensive but fully automatic:

And another, simple one (by Generikb) that doesn't require any resources but building blocks:
_{You might want to ignore the suggestion to put furnaces there. Smelting doesn't give much experience now.}

If you want to make it even simpler but a bit more efficient, you could just put the spawner in a big solid box, then put one line of fence gates and an empty line below it. That way you'll be able to hit the Blaze and they won't be able to hit you.

This could be a problem with your computer. As @creulcat said, Please include more information in your question.

This might work, but I don't know about you:

• Option 1: Please turn off your computer, leave it to cool off, turn it on, open Minecraft, comment if it worked.
• Option 2: If Option 1 didn't work, try this: Launch Minecraft with more RAM, comment if this worked. Example: If your PC has 4 Gigabytes of RAM, then launch Minecraft with 2 GB of RAM.
• Option 3: If Option 2 didn't work, close all apps, Yes ALL of them. even File Explorer, and then run ONLY Minecraft, then load your world. Comment if it worked.

Information: Minecraft runs at 20 ticks per second, if your PC is unable to keep up with this speed, things will be slower in the game. Like blocks will break after 1-5 seconds, Mobs will respond after some time, You know what I mean.

Source: Minecraft Wiki - http://minecraft.gamepedia.com/Tick - "Minecraft's game loop normally runs at a fixed rate of 20 ticks per second, so one tick happens every 0.05 seconds. An in-game day lasts exactly 24000 ticks, or 20 minutes.

However, if the computer is unable to keep up with this speed, there will be fewer game ticks per unit time. As the vast majority of actions are timed based on tick count rather than on wall clock time, this means that many things will take longer on a slower computer."