Hi Jaap!
Some questions for you:
What version of dagster are you running?
What are you using for your dagster storage (postgres/mysql/sqlite)?
Can you confirm, each run of your job is emitting 1000s of asset materializations?
Are you seeing that just loading that run view is taking a long time, or that if you load a run in one browser tab, all requests hitting the same dagit instance are taking a long time?
Also wondering what things look like in terms of resource utilization, using a profiler.