Mac or Windows? What version?
What exact version NUMBER of Lightroom are you using (x.y.z)?
Do you have the GPU usage turned on in the Preferences dialog, Performance tab?
I use lightroom 6 with windows 7 64bit , with Nvidia GT740 video card , but the problem is that there is 2/3 days , until now everything is ok, intel i5 8g ram
You answered 1 out of Maniacjoe's 3 questions. Please answer the other 2. In particular, try turning off "Use Graphics Processor" on the 'Performance' tab of the Preferences dialogue.
I tried to switch off the graphics processor , and is a little bit better , but until a week ago I was using the processor was all right , why? The version is Versione Lightroom: 6.4 [ 1060496 ]