FYI Cyberlink only actively patches products for about 18 months, and PD19 is either already off the list or will be there shortly. There definitely will not be any updates for PD15.
You may have an entirely different reason for lagging than the OP of this thread, and if you'd like forum volunteers to try and help we'd need quite a bit more information.
To start, please follow the steps in the Read Me Before Posting
guide and attach the DxDiag test results so we can see the exact technical details of your system.
Next, attach a screenshot of your timeline and/or providfe a description of the kinds of edits you've made. A screen recording would also help. Things like video speed or direction changes, certain FX and even flipping or mirroring the video can take serious computional power and few, if any, systems would be able to preview them in real time for an extended period.