Onepage (checkout) is very slow

Profile, Profile, Profile.

• xDebug
• Tideways
• Blackfire
• New Relic

Obviosuly do you have serious performance problems, which you hide behind

different AJAX and Caching extensions

Fix this.

The main problem with the speed in the checkout is the cache module. Magento use a cache module witch use the Zend_Cache library and create a huge numbers of files that makes the site terrible slow. You can easy replace it with this:
Cm_Cache_Backend_File
or even better, magento is prepared to work with Redis. With this feature the cache goes to RAM instead the File System. It's a little more tricky but, like I said, Magento it's prepared to use it so it.
Good luck!

here is possible reasons for slow magento checkout:

• magento has to query too many shipping gateway URLs to collect shipping rates. disable UPSP, UPS, Fedes, DHL shipping methods if you don't use them.
• you are using magento 1.9.0.1 that means new order email is sent during checkout - check how fast is that. if mail server is down customers will wait forever after they click on submit order button.
• magento gift message extension adds its 2 cents to checkout page load time. disable it if you don't need it.

you don't need fancy profilers mentioned above - enable default magento profiler or use simple Aoe Profiler and check the trace - it could be some third party extension observers slowing you down.

See these 7 fixes for slow magento site as well.