I just finished Core6k about a month ago. Here was my progression (during full-time study + part-time work):
0-2000: 12 months. Lots days didn't add any. Got behind with reviews a lot, etc.
Took a break for a few months. Read lots of Naruto.
2000-3500: 8 months or so. Whole weeks where I didn't add anything. Started getting really sick of it.
3500-6000: 3 months.
How did I speed up so much for the last 2500?
1. Just sucked it up and did 25 a day. Only had a handful of days during this time where I didn't add or didn't do my reviews. It just became a (pretty much) non-negotiable part of my day. Usually only took about an hour and would do it first thing in the morning, or last thing before sleeping.
2. Suspended (and eventually deleted) cards that I just didn't understand or kept getting wrong. THIS HELPS. I only had to delete about 25 cards in the end, but it stopped me getting frustrated and slowing down during reviews.
3. Focus on the target vocab for that card. Originally I tried to make sure I knew EVERY word perfectly on each card and would fail if I didn't. But then I later realised that those other words come up as target vocab with their own sentence later.
4. It get's easier. It's true. You really do get a feel for the Kanji and readings and can often guess the reading and/or meaning of new vocab as they appear.
I wanted to give up many time too. But as someone who felt the same, also took a long time, but eventually GOT IT DONE. I think it's worth finishing.