My thoughts:

one thing that I think contributed to the slog is that Play by post doesn't really suit linear exploration. Having everyone do a post of "I go north" is a bit of a slog if north is the only way to go. on table top the decision to take everyone north takes like 5 seconds, where as here it takes a week of people posting basically the same thing.

I'd suggest focusing more on rooms if you use the dungeon rout. so like the raptor swamp and Russian camp are good examples of situations to be solved that everyone can get involved with somehow, either though discussion or action, where as the first areas of the bridge and forest where basically box ticking of "we search the one searchable area" or "we go forwards" which are both kinda busy work that only really the first few people can put a creative spin on. 

it might be a good idea also have loot rolls be rewards for clearing encounters instead of environment interactions? or rather you could automate the searching of an area after it is made safe (with the danger passed you take some time to explore the area. X finds a rifle in a closet, Y finds some herbs growing outside, etc.) Which lets you pre-prepare the item rolls/descriptions while you are waiting on them to solve the area. 

Co gms to help manage separate groups or assist in item rolling/description writing might also be helpful as if its only one person you can have them roll for others but not themselves and you can also have them run their rolls/posts past you for approval before it is permanently stamped on the IC tab.