performance: {ts 'datetime'} vs DATEADD()
I recommended (originally, in the comments) that HEDMON try some different dates. Since his test date and his production dates were different, it seemed like it might be possible that there was a selectivity issue with the dates; that SQL could return the data much faster with a more recent date than with an older one.

HEDMON did try that, and found (from his comments) that dates from 2017-02-25 forward responded quickly, while dates older than that were slow.

With this, he felt comfortable that the presentation of the date (using the {ts } syntax) had nothing to do with the issue.