Slam in the Ram.
The Mac isn't emulating Windows - it is running a real genuine, licensed copy.
The emulation is creating a wintel CPU - an old wintel CPU at that and thus based on a structure that is pre gigahertz plus processors.
If the mac's memory is entirely subsumed by pretending to be a CPU, the operating system and program you're trying to run is being mainly done by paging to and from disk.
At some point all of you had had a system config or program where that happened and it's not nice.
Buckets full of memory plus the other advice given is the way to go.
Regards
Rob