A view is just a query that pretends to be a table, so it will come down to the complexity of that query. Each time you're querying the view it will be running the combination of the user's query against the view's query so the performance comes down to whether your DB is optimized around basically "SELECT field1, field2, field3 FROM (SELECT * FROM data_stuff WHERE customer_id=x)". Whether you execute that query as a view or as ad-hoc SQL doesn't make a difference itself.

"Your side" of this can be optimized easily enough, but the user-submitted queries are likely to be inefficient or miss indexes, which is why one database per customer can be better since they each have their own resources.

You can create the views and accounts as needed and destroy them when sessions end rather than keeping them permanently too, so when the user signs in you create the view and account, after the session or some period of inactivity you remove them.