Mac or Windows? What version?
MacOs Sierra 10.12
This bug has now been acknowledged. The team is investigating.
Any updates to the issue will be shown here: Lightroom 6: Tone curve is insensitive to MacBook Pro touch pad | Photoshop Family Customer Community
It would help if you could add your voice there and your System Info (Help > System Info > Copy or Save As) to provide the team with more info.
This should be fixed in the CC2015.8/6.8 Update. Give it a try and let us know.
I have similar problem. After installing 2015.8 curves became VERY slow and unresponsive, before 2015.8 they was slow but usable. Now it almost impossible to use it.
QuadCore AMD Athlon II X4 620, 2600 MHz (13 x 200)
8187 MB (DDR3-1333 DDR3 SDRAM)
AMD Radeon R9 270X (Curacao) (Asus R9270X)
Our team has acknowledged this issues and we are working on it.
Please post your Lightroom>Help>System info here Tone curve still broken in LR CC2015.8 | Photoshop Family Customer Community