Lightroom 6.2.1 when I import photos, The computer is very slowly, an when I shal devenlop the photo the Computer I can't, and I must restart Computer again, and now work it.
First please make sure your Computer meets minimum requirements to run Lightroom System requirements for Adobe Lightroom for Mac and Windows OS
Where are you importing the Pictures from?
Un-check the Option Show Add Photos Screen under Preferences>>General
Also uncheck the "Use Graphics Processor" under Edit>>Preferences>.Performance
Then import to a new catalog in Lightroom
Let us know if that helps