Q: Sometimes, the One Page Magento Checkout loads only the first step without any other information; sometimes, the one-page-checkout loads perfectly and sometimes nothing happens (white screen).
A: The memory limit is being reached from time to time. Try to increase the limit.
Q: Checkout get HTTP Error 500
A: This is not a extension bug. There most likely memory_limit is too low and has to be increased up to 512MB
Why you need to increase memory_limit?
Magento require memory_limit no less than 256Mb (preferably 512)
The cause is because PHP has exhausted the maximum memory limit as set by the configuration value memory_limit.
You would not find any errors in Magento logs. You can check the Apache log to get the confirmation.
Increasing this value can be done either by asking your host to increase the global memory_limit in the servers php.ini configuration file. Another option is to increase this via .htaccess file.
Magento comes with a default .htaccess file in the root directory that includes this configuration directive
php_value memory_limit 64M
php_value max_execution_time 18000
You can change this value up to 128MB depending on your needs. This value should be kept to the minimum necessary to run your application.
Note. Theme itself does not make Magento slow.
The checkout process is extremely critical for online business. Simple and fast OnePage Magento Checkout reduce checkout process from 6 steps to 1. It is a rock-solid, well documented, thoroughly tested extension.