After installing, my magento2 is extremely slow though I did not install sample data?

I see other guys also have faced the same problem (https://github.com/magento/magento2/issues/3123).

Actually, magento 2 default mode and developer mode are very slow. According to @tecjam, we can improve speed if we are on product mode. But with other modes it's still slow.

Please try to this commandline in magento2 directory :

php bin/magento setup:static-content:deploy

With all caches are enabled and static contents are deployed, it might improve speed of site.

First check which deploy mode you are running:

php bin/magento deploy:mode:show

if in developer all files will be generated on each page load. To speed it up,

php bin/magento deploy:mode:set production

this should also recompile the files for you

Disabling JS and CSS merging while developing helps:

Stores > Configuration > Advanced > JavaScript Settings / CSS Settings

You can also enable caching for the things you are not currently changing.