Notebook for data science
Product Manager from Deepnote here.
I'm sorry it doesn't perform well when the notebook is too big. We're aware of the issue, and it's quite hard to solve, but I'll stress its importance to our engineers.
I'm not sure how Datalore handles large notebooks. When you compare them, they both have some nice features (collaboration, secure integrations, reactivity). Deepnote has a few extra ones (automatic versioning, diffing, comments...), and some great ones coming soon (scheduled reports, published applications). On the other hand, JetBrains is a company with decades of experience in the IDE business.
Some other ones you can check which have an open free tier and real-time collaboration: Nextjournal, CoCalc, Databricks (community edition). There are a few more similar products listed here.