To change the font, select the appropriate modifier assigned to the text frame, and then in the "Geometry Node Editor" window, find the "String To Curves" node (it is placed in the foreground) and simply select a new font.

This problem is known to me and results directly from the way geometry nodes calculate text objects when they are scaled, unfortunately, at the moment geometry nodes are not very well optimized when it comes to handling text objects.
However, what you can do is either use geometrically very simple fonts or reduce their geometries using the "Set Spline Resolution" node: