is it just me, or is there a way to improve the frame rate and performance of S.T.A.L.K.E.R games?

I find playing on high detail settings, the performance is very poor and I can't shoot or kill enemies. But if I turn to low details, low resolution, then I can kill enemies easily.

I have Core™ i7-8700 - 6-Core 3.20GHz, 4.60GHz Turbo
16GB (2x8GB) DDR4/3000mhz
GeForce(R) GTX 1070 Ti 8GB
SSD m2

so am sure my PC is powerful enough.

Even on lowest graphic settings, S.T.A.L.K.E.R sometimes has performance issues with camp fires, etc.

Is this normal? Or is it because the game is using onboard 2D graphics chipset?

Stalker can be pretty demanding sometimes.
If you play in DX11 mode — switch to DX10. Set detail distance slider to center. Disable antialiasing if you have it enabled.

Your experience with the differences in detail settings matches mine, so yes, you are using integrated graphics and should use Settings to change the graphics device for the games. Another bit of evidence is the less-than-stellar shader support; I had to use EggChen's Fire Tweak for SoC to keep fire effects from tanking the game response when in view.

Intel keeps making progress with its onboard graphics support, but that Nvidia GTX 1070 should rock at the higher settings.

With your listed hardware you should have no performance problems running any of the Stalker games, baring bugs.

For the fires causing the game to stutter, the EggChen mod is worth trying, but I've found the best solution is to disable all vsync options (both ingame and in the display driver control panel), then use D3DOverrider to apply vsync. Then use display drivers to limit Pre-Rendered frames to 1 (on AMD it's often called Flip Queue Size).

Disabling sun rays and grass distance helps a lot with performance. Also download the '4gb patch' which forces the game to use more than one core. Installing the ZRP mod will fix a lot of game and performance bugs too.

A Geforce 1070 is more than enough to run the game with such settings, even a 1060 can run the games on high settings with extensive graphics mods.

The "4gb patch" does not do anything related to CPU core affinity, it simply alters a flag in the exe allowing it access to a bit more memory.

Dynamic lighting setting is a harverster of resources. If you want performance, switch to static lighting. If you switch to static lighting, you may want to ramp up the brightness, depending on your evaluation, as things may appear a little bleak in comparison, but it takes to become used to. Mind there are two screens of graphical display settings in 'Shadow of Chernobyl', primary and advanced. With static lighting, it should be more feasible to play on very high settings, just mind to drop antialiasing entirely, as it is another resource hog.

I was having stutters last year when the map would load but then I bought new mobo/cpu/ram/gfx card and it's gone. No idea what fixed it. I figured it was just the engine.

Very late but the official multi patch smoothes performance and forces vsync, as far as I can tell it is not included in the Steam or GoG releases. Worked on my copies of CoP (Steam) and SoC (GoG). Now runs almost stutter-free and without constant tearing on my i7/1060.

Just search "Stalker multi patch" and it should pop up on softpedia.