With new Linux kernels, during the release cycle there is typically a release candidate made available by Torvalds once a week, with most cycles including six to eight release candidates. There were nine release candidates for the Linux 4.15 kernel, which makes it the longest cycle since Linux 3.1 was released in 2011. The Linux 3.1 kernel was delayed in part due to the 2011 hack of the kernel.org development server.
As it turns out, the Linux 4.15 kernel delay was also due to security related issues.