It's likely the slowdown you are experiencing with Select is down to a lack of available RAM on your computer. Select typically only uses about 4-5% of your RAM so if you have a look at what else you are running and close anything unnecessary it should run much more smoothly.
Typically applications like Lightroom, Photoshop and Chrome (particularly if you have many tabs open) take up a lot of RAM, so closing them is a good place to start. To see what else is running and taking up RAM, open up your Activity Monitor (Press Cmd+spacebar and type ‘Activity Monitor’).
Select has higher memory requirements in order to run smoothly due to its architecture compared to other more traditional applications. We’re working on improvements and optimisations in this area which may help, but ultimately it’s a finite RAM issue which we can’t control.