new imac is very slow running mojave

I suggest that you create a new user account. Log out of your existing account, and then, log back in with the new account. Then try using your new iMac for a day or so. Any improvement? If so, then there is something amiss with your old account when you restored it to your new Mac. If not, then there may be something else wrong with the iMac itself. Please post back your results.

Did you purchase iMac directly from Apple or an Apple Authorized Reseller or from another third-party source? Is this Mac "new" or just new for you? Did you restore any data from a previous Mac or is this iMac only running the initial software that it came with?