I have noticed that some brushes have become really slow in builds ( probably after the Vc  1.2 transition) first  i thought this may be because the brushes had complex tip, but then i chose basic brushes like Basic-tip-default to test . I tested the same brushes in 2.9 and 3.0 with same settings. I have not enabled instant preview to test the base speed and not the preview (Note below)

The results showed that there is a noticeable speed difference in basic brushes, you can check the screen capture here -> https://dl.dropboxusercontent.com/u/37777793/new-lag.mp4 
(I will attach this video to this bug report too)

as you can see at the end of video the 300px brush takes too much time in 3.0 while the same brush with same size completes the stroke way before it gets completed in 3.0  

Now the reason to disable instant preview was that I wanted to see the difference in real stroke and beside enabling instant preview gives on more side effect since the brush takes too much time to load the stroke is not finished in the background and it really hampers painting , often if you paint something and then decide to delete or clear the canvas after 3-4 sec krita hangs a bit as the stroke is still rendering in background.

Reproducible: Always

Steps to Reproduce:
1.Open krita create a new document switch off the instant preview ( this is to notice the actual speed of the brush)
2.Choose brush basic -tip-default and adjust the size above 120px
3. drawing on canvas with a reasonable speed

Actual Results:  
the brush stroke takes too much time to complete even if it is normal basic brush 

Expected Results:  
The performance of the brush should be onpar of what we had in 2.9 if not faster than it .

Pardon me for setting this bug to major as this hampers the painting process and I think this shouldn't go unnoticed , if you feel there should be a lower priority to this report please change it.