do you use mods? Can you try also disabling the sounds?

[sound]
SoundUse=1 <------ change this to 0

This might help.
Disable onboard sound after popping in a decent (Creative, Asus, Auzentech) PCI-e sound card.

Open IL2 1946 folder. Go to the RCU file. Open it with notepad. Enter "@fps START SHOW" without the quotes.

no.... in the game press shift+tab and you'll open the console, then type fps START SHOW to show the fps

try another core of your Prozessor
In conf.ini delete the ";" before
;ProcessAffinityMask=1
possibles values:
1 = force core 1
2 = force core 2
4 = force core 3
8 = force core 4
3 = force cores 1 + 2
15 = force cores 1,2,3,4

As alternative you can change your Energy saving options.

Can you post your conf.ini?

Yes. After editing the rcu file, you will see your FPS (fps/avg/max) displayed on your monitor.

you don't need to edit any file... you can just type it as I said...

That's also a method. The method I described will give you constant information about your FPS. It's what somebody prefers.

I really bet, its the soundcard/sound-settings.
Your conf.ini posted here, really could help.

here it is

[il2]
title=Il2-Sturmovik Forgotten Battles
hotkeys=HotKey game

[window]
width=1680
height=1050
ColourBits=32
DepthBits=24
StencilBits=8
ChangeScreenRes=1
FullScreen=1
DrawIfNotFocused=0
EnableResize=0
EnableClose=1
SaveAspect=0
Use3Renders=0

[GLPROVIDER]
GL=Opengl32.dll

[GLPROVIDERS]
Open GL=Opengl32.dll
DirectX=dx8wrap.dll

[NET]
speed=8000
routeChannels=0
serverChannels=23
localPort=21000
remotePort=21000
SkinDownload=1
serverName=No Name
serverDescription=
remoteHost=
localHost=
socksHost=
checkServerTimeSpeed=1
checkClientTimeSpeed=0
remoteHost_000=

[MaxLag]
farMaxLagTime=10.0
nearMaxLagTime=2.0
cheaterWarningDelay=5.0
cheaterWarningNum=-1

[chat]
region=(dx=0.83285713,dy=0.07619048,x=0.0,y=0.0)
adr0=ALL
adr1=MY_ARMY
msg3=g35\u0020th\u0020please\u0020take\u0020spitre ece\u0020take\u0020blenheim\u0020please\u0020
msg1=\u0020have\u0020one\u0020frame\u0020per\u0020 second\u0020
msg2=con\u002028.8\u0020please\u0020
msg9=oke\u0020wood\u0020
msg0=making\u0020a\u0020big\u0020brown\u0020bomb\u 0020
msg5=back\u0020to\u0020hl\u0020?

[game]
Arcade=0
HighGore=0
mapPadX=0.67642856
mapPadY=-0.022857143
viewSet=43
Intro=0
NoSubTitles=0
NoChatter=0
NoHudLog=0
NoLensFlare=0
iconTypes=3
eventlog=eventlog.lst
eventlogkeep=0
3dgunners=1


[HotKey game]
PrintScreen=ScreenShot
P=pause
Pause=pause

[HotKey gui]
Escape=activate

[HookViewFly Config]
timeFirstStep=2.0
deltaZ=10.0

[HookView]
MouseLeft=Len

[HookView Config]
AzimutSpeed=0.1
TangageSpeed=0.1
LenSpeed=1.0
MinLen=1.0
DefaultLen=20.0
MaxLen=80000.0
Speed=6

[HotKey builder]
MouseLeft=objectMove
MouseRight=popupmenu
Enter=freeView
Shift MouseLeft=worldZoom
Alt MouseLeft=select+
Alt MouseRight=select-
Alt Ctrl=unselect
PageDown=change+
PageUp=change-
End=change++
Home=change--
Ctrl MouseLeft=insert+
Insert=insert+
NumPad-0=insert+
F=fill
Ctrl MouseRight=delete+
NumPad.=delete+
Delete=delete+
Backspace=cursor
Tab=cursor
F10=land
F11=onLand
NumPad-=normalLand
NumPad+=toLand
NumPad-5=resetAngles
NumPad-8=resetTangage90
NumPad-7=stepAzimut-5
NumPad-4=stepAzimut-15
NumPad-1=stepAzimut-30
NumPad-9=stepAzimut5
NumPad-6=stepAzimut15
NumPad-3=stepAzimut30

[MouseXYZ]
F1=SpeedSlow
F2=SpeedNormal
F3=SpeedFast
MouseRight=XYmove
F4 MouseRight=Zmove
MouseMiddle=Amove
F5 MouseRight=Amove
F6 MouseRight=Tmove
F7 MouseRight=Kmove

[MouseXYZ Config]
RealTime=1

[HotKey Console]
Shift Tab=Activate
[Console]
HISTORY=1024
HISTORYCMD=1024
LOAD=console.cmd
SAVE=console.cmd
LOG=0
LOGTIME=0
LOGFILE=log.lst
LOGKEEP=0

[sound]
SoundUse=1
SoundEngine=1
Speakers=1
Placement=0
SoundFlags.reversestereo=0
RadioFlags.Enabled=1
RadioEngine=2
MusicVolume=10
ObjectVolume=14
MusState.takeoff=0
MusState.inflight=0
MusState.crash=0
MusFlags.play=0
MasterVolume=14
Attenuation=9
SoundMode=2
SamplingRate=2
NumChannels=2
SoundExt.occlusions=0
SoundFlags.hardware=1
SoundFlags.streams=1
SoundFlags.duplex=1
SoundExt.acoustics=0
SoundExt.volumefx=0
SoundFlags.voicemgr=0
SoundFlags.static=1
VoiceVolume=7
Channels=2
SoundExt.extrender=0
SoundFlags.bugscorrect=0
SoundSetupId=8
ActivationLevel=0.02
Preemphasis=0.85
RadioLatency=0.5
AGC=1
PTTMode=1
SoundFlags.UseRadioChatter=0
SoundFlags.AutoActivation=0
RadioFlags.PTTMode=0
RadioFlags.PlayClicks=0
ActLevel=0
MicLevel=0
SoundFlags.forceEAX1=0

[rts]
ProcessAffinityMask=14
mouseUse=2
joyUse=1
trackIRUse=1
DisableIME=0
locale=
JoyProfile=0

[rts_mouse]
SensitivityX=1.0
SensitivityY=1.0
Invert=0
SensitivityZ=1.0

[rts_joystick]
X=0 1 4 9 16 25 36 49 64 81 100 0
Y=0 1 4 9 16 25 36 49 64 81 100 0
Z=0 10 20 30 40 50 60 70 80 90 100
RZ=0 10 20 30 40 50 60 70 80 90 100 0
FF=0
U=0 10 20 30 40 50 60 70 80 90 100 0
V=0 10 20 30 40 50 60 70 80 90 100 0
1X=0 10 20 30 40 50 60 70 80 90 100 0 0
1Y=0 10 20 30 40 50 60 70 80 90 100 0 0
1Z=0 100 100 100 100 100 100 100 100 100 100 0 0
1RX=0 100 100 100 100 100 100 100 100 100 100 0 0
1RY=0 100 100 100 100 100 100 100 100 100 100 0 0
1RZ=0 100 100 100 100 100 100 100 100 100 100 0 0
1U=0 100 100 100 100 100 100 100 100 100 100 0 0
1V=0 100 100 100 100 100 100 100 100 100 100 0 0

[Render_DirectX]
TexQual=3
TexMipFilter=2
TexCompress=0
TexFlags.UseDither=1
TexFlags.UseAlpha=0
TexFlags.UseIndex=0
TexFlags.PolygonStipple=1
TexFlags.UseClampedSprites=0
TexFlags.DrawLandByTriangles=1
TexFlags.UseVertexArrays=1
TexFlags.DisableAPIExtensions=0
TexFlags.ARBMultitextureExt=1
TexFlags.TexEnvCombineExt=1
TexFlags.SecondaryColorExt=1
TexFlags.VertexArrayExt=1
TexFlags.ClipHintExt=0
TexFlags.UsePaletteExt=0
TexFlags.TexAnisotropicExt=0
TexFlags.TexCompressARBExt=1
TexFlags.TexEnvCombine4NV=0
TexFlags.TexEnvCombineDot3=0
TexFlags.DepthClampNV=0
TexFlags.SeparateSpecular=0
TexFlags.TextureShaderNV=0

HardwareShaders=0

Shadows=2
Specular=2
SpecularLight=2
DiffuseLight=2
DynamicalLights=1
MeshDetail=2
VisibilityDistance=3

Sky=2
Forest=2
LandShading=3
LandDetails=2

LandGeom=2
TexLarge=1
TexLandQual=3
TexLandLarge=1

VideoSetupId=4
ForceShaders1x=0
PolygonOffsetFactor=-0.15
PolygonOffsetUnits=-3.0

[Render_OpenGL]
TexQual=3
TexMipFilter=2
TexCompress=2
TexFlags.UseDither=1
TexFlags.UseAlpha=0
TexFlags.UseIndex=0
TexFlags.PolygonStipple=1
TexFlags.UseClampedSprites=0
TexFlags.DrawLandByTriangles=1
TexFlags.UseVertexArrays=1
TexFlags.DisableAPIExtensions=0
TexFlags.ARBMultitextureExt=1
TexFlags.TexEnvCombineExt=1
TexFlags.SecondaryColorExt=1
TexFlags.VertexArrayExt=1
TexFlags.ClipHintExt=0
TexFlags.UsePaletteExt=0
TexFlags.TexAnisotropicExt=1
TexFlags.TexCompressARBExt=1

TexFlags.TexEnvCombine4NV=1
TexFlags.TexEnvCombineDot3=1
TexFlags.DepthClampNV=1
TexFlags.SeparateSpecular=1
TexFlags.TextureShaderNV=1

HardwareShaders=1

Shadows=2
Specular=2
SpecularLight=2
DiffuseLight=2
DynamicalLights=1
MeshDetail=2
VisibilityDistance=3

Sky=2
Forest=2
LandShading=3
LandDetails=2

LandGeom=2
TexLarge=1
TexLandQual=3
TexLandLarge=1

VideoSetupId=17
Water=4
Effects=2
ForceShaders1x=0

PolygonOffsetFactor=-0.15
PolygonOffsetUnits=-3.0
[QMB]
PlaneList=1