If Teams is slow when attempting to join a call/meeting, try the following:
Click on the 3 dots in Teams next your initials/photo and select Settings
Select General and ensure that the Disable GPU hardware acceleration and Register Teams as the chat app for Office options are ticked.
Quit and relaunch Teams.
If you still have issues, try clearing the Teams cache:
Fully quit Microsoft Teams and ensure it's not still running in the toolbar at the bottom right of Windows.
Navigate to %appdata%/Microsoft/Teams in a File Explorer window.
Select everything in the folder.
Right-click on the selected items and delete everything.
Open Teams and wait for it to download required files.