Getting Minecraft not to lag?

If you are running the actual demo on your computer, and not just the on-line classic creative mode, try pressing F3 which gives you various information including fps and tells you when chunks are loading. As you first explore new territory your computer needs to generate new chunks (16x16 blocks) of everything from sky to bedrock (although, primarily what is below or around the level you are at which is the Y coordinate shown in F3). It saves that data occasionally and is also randomly generating mobs (animals in the light and monsters in the dark).

Overall it depends what video you are using (integrated Intel tends to be slower than others) and CPU (cores or threads and speed) and how much RAM you give it. I think the default is 1 GB, but in the launcher you might try changing that to 2 GB (assuming you have at least 4 GB RAM and 64-bit Windows.

Also note that the default Internet Explorer is typically 32-bit, so for 64-bit Windows you likely have to go to java.com and manually install 64-bit Java for 64-bit Windows. That may help speed it up some.

Minecraft is a game that requires a lot of RAM, espcially if played with mods and stuff. 1.6 GB RAM is not a lot. Perharps you can check to upgrade it.