Where are you in the game? (which map)
Did this start happening after the last update, or moving to the next area, etc?
If you load earlier saves, if available, do you have any performance problems?
Try clearing the PS4 cache: press and hold the PlayStation button on the controller, select 'Turn Off PS4', wait another 30 second after it finishes shutting down, and start it again.
If you start the game from a different PS4 account, if available, and start a new game, do you have any issues in the tutorial ship or the start of Fort Joy? Another account would give a separate drive space for saves.
If you start a new game in your current account, can you check the performance there? Moving a bit from the initial character position can trigger an auto save, which might overwrite one from your current playthrough. If you don't already have an honour mode difficulty save, starting one would avoid that problem, since it uses a separate, single save slot (in the difficulty list, after selecting Tactician there is a choice between Regular and Honour).
If a new game or older saves are fine, but your latest saves still have this problem, please email supportdos2@larian.com with your latest save(s) (or a Dropbox or Google drive link, etc); they would need to be transferred to a USB flash drive first.
In the Saved Data in System Storage section of the Settings menu, click Copy to USB Storage Device, select the relevant save(s) and Copy. From there, on PC you can zip the files, then copy to email or upload somewhere and provide a link.
