How to Say "heavy going" in Spanish
The Spanish word for “heavy going” is “ladrillo” — B2 level.

Examples
Ese libro de historia es un ladrillo.
That history book is a real bore (heavy going).
La conferencia fue un ladrillo y me quedé dormido.
The lecture was a total drag and I fell asleep.
No veas esa película, es un ladrillo de tres horas.
Don't watch that movie; it's a three-hour bore.
Using with 'Ser'
To describe something as boring using this word, always use the verb 'ser' (to be) because it's considered a characteristic of the thing.
The Indefinite Article
Even when you use it as an adjective in English (it's boring), in Spanish you keep the 'un' (it's 'a' brick).
Using 'Estar'
Mistake: “Saying 'La clase está un ladrillo.'”
Correction: Say 'La clase es un ladrillo.' We use 'es' for things that are boring by nature.
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