The default settings for lightmaps are 64. Oftentimes this is too low of a lightmap resolution for walls or any large pieces. Increasing that size is common practice. When you setup your second UV channel to handle lightmaps some considerations you want to make are that you’re using the UV space effectively by using the entirety of the space but still giving a buffer between UV islands to make sure there is no shadow bleed.
As for the light rendering, yes it can take a while. CPU and Memory will definitely play a role here! Also, if you’ve not set up a lightmass importance volume you’ll see significantly longer build times. The ligtmass importance volume should encompass you’re playable area. Make sure this is being used in that area. This will tell the engine that anything within that space should be given priority and anything outside will have a lower quality shadows.
Depending on the size of your map, using culling volumes and LODs with your meshes will definitely improve performance as well.
If you have any questions feel free to ask!