I think its because of old hdd and small amount of ram (only 4 gb)?
Can I simply copy wallet.dat from old pc to my iMac (with SSD and 12 gb of ram) to make the process faster?
If you decided to do that, you should increase the database cache of Bitcoin core in your iMac to about 6GB to further increase the sync speed.
The option is in "
Settings->Options->Size of database cache" just below 'prune', setting it to 5722 should provide significant sync speed boost.