Product and version
all FMP Versions from 14 to 16
OS and version
macOS 10.13.1 and 10.13.2 High Sierra
Documented here is an error of FMP under High Sierra:
If the drop down arrow for a value list is hidden in the inspector, the field will take keystrokes very slowly. This occurs only if the entire field content is selected (which is the default behavior when clicking into the field).
When repositioning the cursor by clicking again or by moving it using the arrow keys, the field suddenly returns to normal speed.
How to replicate
Use a value-list and turn off the arrow symbol in inspector.
Or see my example file that shows the error. The best way to test ist is by typing fast "wild" keyboard inputs with all 10 fingers.
In versions prior to FMP16.0.3, the effect is even worse.