Someone tell me I am missing something obvious... Just played the first Rimtech mission which seemed generally pretty simple. Build two combots and prosper. Only issue is, it took me at least 30 minutes? The build times are absurd on those parts. Am I doing it wrong and missing something? or do people commonly build multiple plants and assembly pads?

Even if thats the case, my tanks and other vehicles took decades to assemble. Are timers just painstaking long in the game or am I foolish? My hover van or whatever were extracting lava three at a time and I built three power plants. Both my red and yellow bars were pretty much full throughout. Frankly, not really sure what either is but irrelevant so long as they are growing.

Try as I might, I failed to find any method of increasing game time as well. Wishful thinking on my part?

So the scenario instanced me to use Diego and I did that. But yes, I also had a cryofarm which I built a team from that sat there until I loaded them into the second combot.

So both the resources bar were full. So I assume the answer is yes. Do they need to be attached to something? I built three energy generators and had three harvesters on the lava. I just plopped down the building and click to build a torso. Easily took 5 minutes real time for one torso. Nothing in the build queue before it, just literally watching the percentage bar crawl from 0-100 %.

Can manpower be applied to other buildings than the combot? Maybe thats it, I certainly didnt assign anyone.

No, manpower is not assigned, it's automatic. You just build it, something like power in C&C or unit limit buildings in AoE or WC3. But it's needed for every building and every non ComBot unit. The resource bar has two lines - how much resorce you have and how much is already used of it. So if both lines are completely full - you should expand your manpower. You should continuously build cryofarms, for simple mission 4-8 should be more than enough depending on the situation.
The issue you are describing is very reminiscent of low manpower, everything just builds much slower, again, like in C&C with power.