lots of possibilities. here are some
lack of memory 2GB minimum
slow memory
older cpu
a lot of Terminate and Stay Resident programs, TSR's don't clear out of memory when done and thus there is less memory available for the program to use.
a slow mechanical hard drive like on a laptop.
viruses and malware
lack of space on your hard drive
also because of the tax law changes the program has become a lot more complex. some numbers can't be calculated in one step but through a series of iterations.
there are other possibilities.
not knowing the specs of your computer, your issue is difficult to diagnose.