How to get SUM from sum of relational field

i have two models Sell and Slip where:

class Slip extends Model
{
    protected $guarded = [];
    protected $dates = ['bill_date'];


    public function orders()
    {
        return $this->hasMany(Sell::class);
    }
}

class Sell extends Model
{
    protected $guarded = [];

    function product()
    {
        return $this->belongsTo('App\Product');
    }

    function slip()
    {
        return $this->belongsTo('App\Slip');
    }

}

in migration I have column for slips table:

Schema::create('slips', function (Blueprint $table) {
            $table->increments('id');
            $table->string('customer_name');
            $table->string('customer_age');
            $table->string('customer_gender');
            $table->dateTime('bill_date')->nullable();
            $table->integer('paid')->nullable();
            $table->integer('discount')->nullable();
            $table->integer('doctor_id')->nullable();
            $table->timestamps();
        });

and for sell:

Schema::create('sells', function (Blueprint $table) {
            $table->increments('id');
            $table->string('product_id');
            $table->integer('sell_price');
            $table->integer('doctor_id')->nullable();
            $table->integer('slip_id')->nullable();
            $table->timestamps();
        });

I have this query:

$slips = Slip::with(['doctor']);
        $slips = $slips->withCount(['orders as total_sales' => function ($q) {
            $q->select(DB::raw('SUM(sell_price)'));
        }]);
$slips = $slips->whereBetween('bill_date', [$from, $to])->whereHas('doctor', function ($q) {
                $q->where('id', request()->query('doctor'));
            });

What I want is to get is the sum of all sell_price from the databse after I filter. I can get sum of sell_price(total_sales) for each slip, but I want sum of total_sales before I do pagination.

I know a way I to do that:

$slips->get()->sum('total_sales')

But if I do this with 10,000 data, it takes 5 min to load which I don't want that. I would like to get the sum from database. Is there any workaround to do that?