The speed effect doesn't affect the length of the clip I'm editing. Thus, slowing down the clip makes is so the end of the clip is cut off. Is there any way around this?
The speed effect could see better days, for lack of better words. You've started off right by doing it in a comp. Have you tried the rate stretch tool then moving it to a comp?
HitFilm needs improvements in the slow motion area. The most common complaint is the conforming since it deletes frames. If you put a 48 FPS video in to a 24 FPS timeline it's done for you, in Premiere for example.
It can be tricky to get it right in HitFilm, but I don't doubt that there's a solution out there, even if complicated until it's improved.
I'm just going to point you to this thread because this is the sixth thread in a week on the topic. Read all my posts here.
No effect will alter the duration of the clip you apply it to, that's not how effects work. Effects apply modifications to the contents of the layer, they don't change the layer itself. If you want to change the duration, you need to use one of the editing tools, such as the Rate Stretch tool, or the Speed/Duration option in the right-click menu.