Diggles has more game settings than you can see in the settings menu.
They're in the Windows registry. You can access this by running the program RegEdit.exe on your computer (comes with Windows)
Caution - if you change random things in the Windows registry you can break Windows or applications! But if you just stick to the below directory you should be fine. But at your own peril etc etc.

In the left panel navigate to:
HKEY_CURRENT_USER > Software > SekOst > Diggles

This directory has all the Diggles settings in. I don't know what all the settings do (its been a loong time), but I seem to remember changing some settings breaks Diggles, so make sure you remember the defaults before fiddling.

Important Note: Any settings you change here while Diggles is running will be reset when you stop Diggles, and I think when reloading too. So stop Diggles before changing these.

Some settings you may want to change:

* Performance > MaxFramerate: set this to whatever your monitor refresh rate is (probably 60). This will make the game smoother.

* Misc > NoGamespeedLock: 1 = allows you to change the game speed using + and - keys (keypad) between 1x, 2x, and 4x speed. 0 = Game only runs at normal speed (default).
Note: Speeding up the game can reportedly do strange things to the gameplay; use with caution.
Note: Game speed returns to 1x when an scripted event happens (i.e. tutorial points).
Note: There is no on-screen warning as to what the game speed is.
Note: The maximum speed you can change it to is Misc > MaxGametimeFactor; probably best to leave that as 4 but 8 works too if you're really impatient.

* Misc > PauseOnDeactivate: 1 = Game pauses when in the background (default). 0 = the game will continue running in the background if you switch to another application (but it'll be very laggy, at least it is for me).

* Misc > ShowFMV: 1 = Show intro movie at game startup (default). 0 = Don't show it.

* Misc > ShowSubText: 1 = Show subtitles. 0 = Subtitle Off (Default)

The old version had the "WigTweak.exe" to set these stuff, not sure if some of these settings could be added with a launcher with some checkboxes or even with the old tool...

*unofficial tool WigTweak.exe. Can been found online if there is some live links.

Talking about registry settings in [HKEY_CURRENT_USER\Software\SekOst\Diggles\Performance] , does anybody know which of them can be adjusted to improve FPS?
It is a bit annoying to have 20-25 FPS in an old game which should run perfectly at 500 MHz CPU while my actual CPU has 3GHz. I tried setting RatingCPU, RatingGfx, RatimgMem, etc to 100 (it was 25 by default) and adjusting MaxFramerate, but without any noticeable effect. Oh, and MaxFramerate gets reset to 200 every time I launch the game.

UPD: Switching off post-processing skyrocketed max FPS to 35-36 when zoomed in, but when zoomed out FPS is still low.