I have only been working at North American tech company for 9 months now. And for most of projects I have worked on till now, I have been super fast and finished my work mostly before deadline/estimate. My question is can this fast work backfire on me? My manager has always praised me but can it be bad like me being disliked by team mates or having problems in future? Should I slow down and finish it slowly (In that case, I am not sure what do I update in project syncs if I have been lazying around and not doing much)? Or continuing at speed will be beneficial for me?
It can be, but it's not very likely.
Here the ways I can see it backfiring on top of the reasons you mentioned:
Both of the above would only happen in an unhealthy team, and I wouldn't solve it by moving more slowly.
If you're feeling some anxiety over this right now, I just want to remind you that you are in a very good position. By being fast at completing tasks, you are unlocking the most important resource: Time. In fact, this is something I have been able to do across my entire career as I have always been an Android engineer and naturally fast at it due to side projects: Implementation work has generally not been a problem for me.
That being said, what's important is how you use that extra time. Here are some ideas stemming from how I used it, all of which are important for growing into a senior software engineer:
I think Alex brought great points and I would also recommend sharing your concerns with your manager. They could give great advices being aware of the specifics and the exact people in your team.
In addition I recommend spending time building relationships with your peers, building informal relationships (by being genuinely interested in them), asking them if there is anything you can help with (in 1:1 fashion to be more genuine) and also sharing things you became great at (e.g. via tech talks or even simply by improving your team's documentation).