Ordinarily, in your scenario, an After Scanning deployment schedule should have the least overhead, as it will generate less total deployment attempts over the same time period. A 15/30 minute schedule will likely cause multiple deployment attempts for the same asset during the course of a single day.
As far as performance impact on your Lansweeper server though, this should be quite minimal, as the number of active deployments that can run at once is limited by your thread count. Additionally, the performance impact of running deployments on your Lansweeper server should be quite low, as deployments are performed by the local task scheduler of your workstations. Deployment thread settings can be modified under Configuration\Server Options.