That is way too long for a regular RAM disk. However, if you are using an image file of such a large size, that sounds about right.
Let's say your SSD supports a sustained read rate of 300 MB/s. Then 192 x 1024 MB / 300 = 655 seconds, or nearly 11 minutes just to read the image file. After that some more time is spent during the memory allocation and initialisation.
It is possible to create a RAM Disk after login, but it still won't be usable until a similar time period has passed. To create a new RAM disk, choose Disk – Add or Disk – Add Boot Disk from the main menu. The former will create a RAM disk initialised when a user is logged on, the latter will create a RAM disk initialised during system startup.