A primary driver of the 'slowness' is a major focus upon safety in the workplace. Take a look back at the death rates per 1000 workers for some of the historical projects that people often like to point to and then look at a modern project.
The second element is that these safety elements are often enforced by regulation. For example, look at how much extra scaffolding is in use today during construction (which used to be ladders).
To directly look at corruption, this is an issue but not in a direct way. Corruption happens through the 'contractor ladder' where the primary contractor has a subcontractor who has a subcontractor who has a subcontractor. Repeat ad infinitum.
One of the primary reasons for this is the challenges in maintaining a large enough workflow to keep a standing workforce employed. It's difficult to justify paying expensive construction workers and engineers when they're not actually building anything.
Finally, tendering protocols are often quite naive and have been implemented to make something "least cost". This has led to a nightmare scenario of companies underquoting in order to win a tender, secure in the knowledge that a government will not leave a project half finished. To remedy this better contracts are required, for example, you can offer a recurring revenue stream (e.g. 20-30 years) to a company in return for a particular project. On the other hand, this can often lead to poorly build projects that last exactly 30 years.