I do not think that font colors and cell merging can cause slowness. These are static values. Merging cells can cause errors when you try to do calculations on them, but if the merged cells are just labels and do not interfere with any formula calculations, they will not slow your workbook.

I also do not believe splitting the workbook up will help, though I am not certain of this at all. The only way to know for sure would be to test it out. I imagine, though, that at least some of your functions will stop working. INDIRECT(), for instance, does not work on a closed workbook. Neither does VLOOKUP. SO I imagine that there would be a bit of work involved before the split workbooks would work properly and you could know for certain whether or not it is faster split.

I imagine your problem is due to array formulas, or formulas like SUMPRODUCT. Sometimes these formulas are necessary and you just have to take the speed hit, but a lot of times it just needs a redesign of your data, and maybe a helper column or two, to speed things back up.

Without a sample workbook I cannot say anything more than general statements.