The major benefit of GPU acceleration in Lightroom 6/Lightroom CC is on 4K and 5K monitors. So while you may see some speed benefit from GPU acceleration on a Surface Pro 3, one place where you do not get a benefit from GPU acceleration is in the use of the adjustment brush, which is actually slower when the GPU acceleration is turned on. Thus, you are probably wise to leave the GPU acceleration off. When it is off, are you seeing lack of speed in the develop module?
Also, Adobe says your GPU need 1GB VRAM for GPU acceleration, and you do not have that (if I am understanding you properly)
Without GPU support performance of the adjustment brush is "acceptable". I bit like in LR5.7, maybe a tad slower
With GPU support performance of the adjustment brush is unacceptable slow.
Intel HD graphic 5000 properties provides the following memory info:
- Total available graphics memory 4176Mb
- Dedicated Video Memory 128Mb
- System Video Memory 0 Mb
- Shared System Memory 4048Mb
I'm using LR6 on a desktop as well with an HD monitor which has lower resolution than a SP3. There the default GPU enabled setting actually increases performance...
There is no way the graphics card in a Surface is powerful enough to see any benefit from GPU acceleration. In fact there are extremely few machines where it consistently leads to improved performance and then only when you never use brushes on your images. Even very powerful machines can be brought to their knees with a few brushstrokes I have found when GPU support is turned on. So my general advice is to turn it off except when you have a really powerful graphics card and are running on a very high res monitor (4K and up).
I agree with Jao vdL, the adjustment brush problems you are experiencing indicate your best choice is to turn off Graphics acceleration.