New iMac (Retina 4K, 21.5-inch, 2019) with Mojave worse than iMac 2012 with OSX Sierra

I just bought an new iMac 2019, the newest and the most updated, however the computer is just as slow as my 2012 model with OsX Sierra, the finder freeze, finder run very slow every time i use , applications crash o run very slow, almost nothing works. I cant open several programs because immediately the performance goes to zero. Is it a problem of all your new iMac or Mojave operating system? Is there any way to fix this or do I have to install some other operating system to the machine? As I mention it is a machine that has not left more than a couple of months and works very badly.