Hello Jonathonr,
We're sorry for the trouble you had with Adobe Reader, what is the version of Adobe Reader you are using in which you are experiencing the lag issue while filling the PDF forms? To identify, please refer to Identify the product and its version for Acrobat and Reader DC
Is it specific to one PDF form or with all the PDF forms?
If it is specific to one PDF form, is it possible to share the file with us so that we can test it at our end? Share the file by direct/private message only How Do I Send Private Message
For testing purpose, please disable the "Protected Mode at startup' and "Enhanced Security' from Edit menu of Adobe Reader>Preferences>Security(Enhanced)> uncheck these two options, click OK, restart the application and check.
Note: Disabling the Protected Mode and Enhanced Security is only for testing purpose, please enable it back.
Install the latest update of Adobe Reader 19.010.20098 from help>check for updates, reboot the machine if possible and check.
Let us know how it goes and share your observation.
Thanks,
Anand Sri.