Loading levels and data precomputation
Every time a level is loaded, D2X-XL does precomputations of a lot of data for it when it doesn't find that data or detects that it is outdated. The required calculations are done on the graphics card to speed them up as much as possible.
For most levels, these precomputations shouldn't take long, but they can take a minute or two for larger levels. This also depends on your hardware, so if you have a slower CPU or graphics card, it may take a little longer. However, it will take nowhere near as long as the lightmap precomputation in v18, don't worry, you should be able to watch the progress bar fill up in real time.
Please note that for some precomputations, there is only a single progress bar step, so if it stops for a while, the program hasn't stopped working, but is very busy. At the end of the lightmap calculation, there may be very little progress for a short while; D2X-XL is saving the lightmaps to mass storage (SSD/HDD), which can take longer than computing them, so this is not a sign of D2X-XL having stopped working.
This data may be precomputed again when you change certain settings that affect them, e.g. lightmap quality or lighting quality. Hardware raytracing speeds lightmap computation up significantly. You can also speed it up by chosing high lightmap quality instead of ultra. You won't be able to notice ultra in any but the most extreme cases (huge segments): The pixel density of ultra exceeds that of the render in most cases, so you won't lose visual fidelity when going for high quality, and computing that is four times faster than ultra high. If your graphics card has less than 12GB VRAM, it is highly recommended to stay at high lightmap quality or below; With ultra even some levels from Descent 2's main campain will cause D2X-XL to use more than 8 GB VRAM, and Windows usually needs around 1GB even when D2X-XL is running, and more if you run other programs (like e.g. a web browser) when using D2X-XL.
Handling program isssues and bugs
If you encounter crashes, please see the crash log post for how to report them.
For all other issues, please make screenshots if the issue is a visual glitch or explains the problem. Please also tell me the level you played and the settings you used (particularly lighting, lightmap and render quality). Create a new thread with a meaningful title in this forum area and post your info and screenshots there.
Graphics Device Lost
Usually this indicates that D2X-XL tries to use more VRAM than your graphics card and system can offer. In that case, try to reduce lightmap quality, since lightmaps are the biggest VRAM consumer, and each quality level down reduces their memory consumption by three quarters. As I said: High lightmap quality is very good - you don't need ultra.