Mongo 3.6 performance issue
As per your system details the readahead is set to 16K. But as per MongoDB BOL  for the WiredTiger storage engine. Set the readahead setting to 0 regardless of storage media type (spinning, SSD, etc.).

Setting a higher readahead benefits sequential I/O operations. However, since MongoDB disk access patterns are generally random, setting a higher readahead provides limited benefit or performance degradation. As such, for most workloads, a readahead of 0 provides optimal MongoDB performance.
In general, set the readahead setting to 0 unless testing shows a measurable, repeatable, and reliable benefit in a higher readahead value. MongoDB Professional Support can provide advice and guidance on non-zero readahead configurations.

As MongoDB documentation for the WiredTiger, consider the following recommendations on Linux environment:


Turn off atime for the storage volume containing the database files.
Set the file descriptor limit, -n, and the user process limit
(ulimit), -u, above 20,000, according to the suggestions in the
ulimit reference. A low ulimit will affect MongoDB when under heavy
use and can produce errors and lead to failed connections to MongoDB
processes and loss of service.
Disable Transparent Huge Pages. MongoDB performs better with normal
(4096 bytes) virtual memory pages. see  Transparent Huge Pages Settings.
Disable NUMA in your BIOS. If that is not possible, see MongoDB on NUMA Hardware.