You could change the infill pattern to triangular, its one of the faster patterns to print, and it doesn't make you loose heavily on mechanical strength (although it does make you loose some). There is a setting on prusaslicer that also helps to speed up big prints, it basically merges infill layers, so that if the extruder permits it, for example, with a 0.4 mm nozzle at 0.16 mm layers, instead of 0.16 mm infill layer, it will print 0.32 mm layers (or more if your extruder is bigger, basically multiples of your chosen layer size). You could also increase the printing speed for internal fill and walls, but maintain the external perimeters speed so that visual quality is not harmed. Additionally, you could change your nozzle to a bigger one, like 1.0 mm, and print with a bigger layer size.
Buying good quality filament is also important, as a good filament is usually easier to print with good results, so it permits you to increase the speed a bit.
Overall, Id say that you shouldn't pay too much attention to the time part, 5 days is ok for a helmet using fdm printing. Resin is much faster for big prints, but printers are smaller so you usually cant print one piece helmets there. Print quality saves you tons of hours of manually perfecting the whole thing, the machine´s time isn't worth as much as yours.
