I'm familiar with that problem, and overcame it by really working at making good cards. Today, I have 6000 cards in my collection, and daily reviews take 15-20 minutes; usually 100-120 cards. (I have been using Anki for a lost time. My collection is almost 10 years old.)
Granted, writing good cards is easy to say and really hard to do. Some specific pointers:
- cards should take less than 10 seconds to answer
- most questions should be 7 words or so
- if your question has multiple clauses, split those up into separate cards
- cloze deletions are great, consider multiple clozes per card
- around 10% of your cards should have images
- good images help a lot, mediocre images make it worse -- when in doubt, don't
- Answer this Question cards should have an opposite Question for this Answer like Jeopardy
In my experience, you really only start to see these problems at the tens of thousands of flashcards level. I have ~70,000 mature flashcards and also have the growing backlog problem, even if I don't add new cards for weeks. My collection is 8 years old.
I agree that making good flashcards helps alot. Another big thing is to make sure the knowledge graph is connected, eg there are no 'orphaned' individual cards / groups of cards. Those tend to suffer seriously from decay for me.
Granted, writing good cards is easy to say and really hard to do. Some specific pointers:
- cards should take less than 10 seconds to answer
- most questions should be 7 words or so
- if your question has multiple clauses, split those up into separate cards
- cloze deletions are great, consider multiple clozes per card
- around 10% of your cards should have images
- good images help a lot, mediocre images make it worse -- when in doubt, don't
- Answer this Question cards should have an opposite Question for this Answer like Jeopardy