A CTOT (slot time) can be applied to a route for an number of reasons and unless you speak to operations you will not know where in your route the restriction is. Firstly, a flight plan, if unrestricted, is for a departure + or - 15 mins of the SDT (-5/+10 for CTOT). You may try and change the departure time but you may them pick up an unfavourable CTOT. If you are unrestricted, as has been mentioned, the arrival airport will expect you somewhere close to the STA. Its resources, and those of the ground agents, are aligned to schedules, they dont have crews just sat around waiting for aircraft to pitch up! On the rare occasions I was running ahead of schedule, I found it best just to wait and have another coffee.