When playing the game, it will sometimes slow down to running at like 10 FPS, which results in tediously slow machine simulation. It seems to be generally related to too many things happening at once, however some objects have a much, much bigger impact than others. It can even slow down during the "construction" phase when hovering over an object (I'm thinking it's because it has to render the dotted outline around the current object).
I'm gonna rule out my hardware being too weak (i7-8700K, GTX 1080) so perhaps the DOSBox is having some problems with the emulation? I am not familiar with DOSBox and the Graphic modes from the "Graphic Mode Setup".

I couldn't find anything on this slowdown/lag problem, so I'm grateful for any help I can get.

Open the file "dosboxEVEN1.conf" in text editor. Find cycles under cpu section and change cycles number. Probably you need to set it higher (maybe 20000). But best is set it to max.