Custom foreign key in relation cast as integer instead of string?

I need to link two tables together using a custom foreign key on both tables, as follows:

public function image_assets()
    {
        return $this->hasMany('App\Models\Asset', 
								'unique_serial_id', 
								'unique_serial_id');
    }

However, the problem is that the resulting query treats 'unique_serial_id' as a string instead of an integer, which slows the query down no end (the underlying field is an integer). I can see from Clockwork that the query looks as follows:

SELECT `unique_serial_id`, `updated_at`, `s3` FROM `assets` WHERE `assets`.`unique_serial_id` in ('13934', '18249', '18250', '18251', '18252', '18253', ...) //there are ca 1000 entries, so I won't list them here.

No amount of indexing will speed this query up (it's currently at 1700ms!). However, it would no doubt be much faster if the resulting value was queried as an integer, matching the column type, as follows:

SELECT `unique_serial_id`, `updated_at`, `s3` FROM `assets` WHERE `assets`.`unique_serial_id` in (13934, 18249, 18250, 18251,18252, 18253, ...)

Is there any way to enforce custom foreign keys to be treated as integers in Laravel?

(Update: I also noticed something else that is strange: when I copy the query from Clockwork into TablePlus, it executes the query in 75ms (which is fine), but in php it takes 1700ms.)