I just got a new pb in a game that is only a few seconds less than my previous PB, and It keeps me in the exact the same number on the leaderboard. I feel like if i submit it i would only be wasting the moderator's time on approving other runs that are actually good. Should i submit it?

You can submit as many of your PBs as you want. Moderators sign up to verify runs, and they can always add more verifiers if they need to.

There was a good discussion on this here recently:

https://www.speedrun.com/speedrunning/thread/1i9br

However it all boils down to - if YOU want to submit your PB, then yes, you absolutely should do it if you want to do it :)

@PoleoGoleo just to add to this, you can submit any pb run you like, and don't worry about the modifiers, they're usually happy to have people running their games. However, if you obtain a world record run, or even a top 3 time in very popular runs, i would recommend submitting your run as soon as you get the chance. This is to avoid hoarding, a technique which is explained in the thread linked above. Although very few games have specific rules banning hoarding, it is considered bad etiquette. Other than that, submit whatever pb's you want. I personally submit every pb, but it's completely up to you.