What am I doing wrong?!?!?

Hello. fairly new to infuse. I am trying to watch movies ive downloaded and they take forever to load and constantly buffer. I have a fully fully loads 2017 27’ iMac, 300mbps internet connection pulling 150-200 on wifi and a 4th gen and 4k Apple TV. any ideas ??

Maybe your network has an issue. You can download the speedtest app and run it. If your internet speed is OK, your local network speed certainly is.
If your local iTunes homesharing videos play OK, then that also means your local network is OK (at least for the bitrate pf these videos).

Try using NFS, which works very well, instead of SMB.
There is a free-to-use app called NFS Manager that makes it quite easy (just share your video folders as read-only with the default options).