I am a bit disapointed to see that Manifold Viewer is very slow to open these images.
You have to say in what format. If you think it is slow you clearly are not using Radian .map format, which opens everything instantly. You must be using a slow format. Slow formats are slow, and fast formats are faster. Super-fast formats like Radian .map are instantaneous. It's true that intrinsic Radian speed can open many slow formats faster than other products. For example, recently there have been some comparisons as to how fast Radian can open (load) various raster formats compared to Arc and others. But there is only so much you can do with a slow format.
It's like the difference between finding an audio file on a cassette tape and the totally instantaneous random access you get with a modern zip drive. You could have the fastest parallel computer on the planet and it is not going to make the cassette tape spin any faster on a cassette player.
.bmp is a slow format, a very old format that is not a good choice for 9 GB images. How long does it take to import a .bmp in ArcGIS Pro? A good example is how quickly Radian can open a big raster in 1/10 second that takes Arc anywhere from 3 minutes to 30 minutes.
A much better choice for 9 GB images is Radian .map format, which opens the target data set in 1/10 second.
Opening these photo in Manifold 8.30 takes hours ... Either opening a manifold file which contains these images
Manifold Release 8 was not designed to run super fast with big images. It was designed to do a good job of smoothly integrating vector and raster function given the sizes of project most people did.
As image sizes went up, Manifold developed Radian. Working with big images requires Radian technology. Use that and both Radian and Viewer will open the file instantly. If you have images in Release 8, open the Release 8 .map with Radian and save it as a Radian project. After that it will open in 1/10th second. That's what you want, true?
If you want super-fast, parallel performance, as discussed in the Getting Started topic (... a good idea to read carefully for useful tips...), as adamw recommends get a friend with Radian to save your big .bmp into fast Radian format. Or buy a Radian license and do it yourself. :-)
Note, by the way, that if you don't want to buy Radian or have a colleague help you use hyper-fast Radian format in Viewer, you still have a useful standalone tool in Viewer. It will open that .bmp faster than something that cost you a few thousand dollars from ESRI, and it will do it for free.