Q: Replaced my hard drive now Macbook is extremely slow.

Download and run EtreCheck, then post the report here so we can learn and discuss what ails your Mac.

Nothing but a hardware fault could cause the symptoms you describe.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider. You may have to leave the machine there for several days.

Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keep your confidential data secure during hardware repair.

Apple recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.

Testing at an Apple Store is free. They'll tell you what your options are. Good luck.