Changing a product.category's valuation from manual to real-time or real-time to manual does mainly two things. The first one is emptying the current stock and valuation. The second is to replenish the stock according to the new valuation.

This process can be heavy when the number of product.products related to the active product.category is big. This can happen when product.attributes are set to "Creation: Instantly" for instance.

This commit aims at improving the overall speed of this change in some cases. A first optimization is to use `product_tmpl_id` to retrieve the `product_variant_ids`. When there are a lot of products, it's faster to explicitely use the delegated field `product_tmpl_id`. This avoids lots of calls to `__getitem`/`__setitem__` in `_compute_related`. The downside of doing this is that subsequent calls to `self.product_variant_ids` are gonna raise a CacheMiss. So we have to explicitely use `product_tmpl_id.product_variant_ids` every time. We argue that it's not really an issue here as retrieving the variant_ids from a product.product itself is not that frequent in the codebase.

A second optimization is to avoid calling `product.qty_available` in `_compute_value_svl` in case `avg_cost = 0`. With an avg_cost of 0, the total_value is always going to be 0. So there's no point in calling the heavy compute method `_compute_quantities` to retrieve `qty_available` here.

#### speedup

In a database with 228 000 product.products linked to the same product.category, the time to switch the category valuation from manual to real-time: +15min (timeout) -> 18s

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Forward-Port-Of: odoo/odoo#231206

To fully comply with regulatory requirements, we need to support an additional category called "Reduced Tax Rate" when a product line has a Reduced VAT rate (e.g., 20%) for sales of goods and services to final consumers when payment is made by debit card or electronic money instrument (and other specific reductions in similar cases).

1. Detecting "Reduced Tax Rate":
* Identify product lines with a VAT rate that is neither 0% (exempt), 10% (minimum), nor 22% (basic). Any VAT rate outside these three should be considered "Reduced Tax Rate".
2. Modifying XML Output:
* In the <Totales> section of the XML, include the total amount of VAT under the "Reduced Tax Rate" in the <MntIVAOtra> tag.
* Example: xml <MntIVAOtra>140</MntIVAOtra>  (where 140 corresponds to the VAT calculated at the reduced tax rate, e.g., 20%).
* For each product line using "Reduced Tax Rate," set the <IndFact> tag to 4: xml <IndFact>4</IndFact> 
        *Ensure the total amount reflects the base amount plus the VAT under "Reduced Tax Rate".
3. Tax Grid for Configuration:
*Add a new tax grid called Sales Reduced VAT to be used for the tax configuration of the "Reduced Tax Rate."
* This will ensure proper reporting and consistency in tax declarations.
* The new tax grid should be selectable when configuring other taxes.
Odoo Implementation Considerations:
* The tax computation logic in Odoo already supports defining taxes at different rates.
* Adapt the XML generation logic to check for product lines with a non-standard VAT rate and apply the necessary modifications. Ensure the final totals in the XML align with Odoo's computed tax amounts.
Task latam side: 1330
Task Adhoc side: 52999

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I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#221073

1) Detecting "Reduced Tax Rate":
* Identify product lines with a VAT rate that is neither 0% (exempt), 10% (minimum), nor 22% (basic). Any VAT rate outside these three should be considered "Reduced Tax Rate". 
2) Modifying XML Output:
* In the `<Totales>` section of the XML, include the total amount of VAT under the "Reduced Tax Rate" in the `<MntIVAOtra>` tag.
* Example: ```xml <MntIVAOtra>140</MntIVAOtra> ``` (where 140 corresponds to the VAT calculated at the reduced tax rate, e.g., 20%).
* For each product line using "Reduced Tax Rate," set the `<IndFact>` tag to `4`: ```xml <IndFact>4</IndFact> ```
    * Ensure the total amount reflects the base amount plus the VAT under "Reduced Tax Rate". 

3) Tax Grid for Configuration:
* Add a new tax grid called Sales Reduced VAT to be used for the tax configuration of the "Reduced Tax Rate."
* This will ensure proper reporting and consistency in tax declarations.
* The new tax grid should be selectable when configuring other taxes. 
Odoo Implementation Considerations:
* The tax computation logic in Odoo already supports defining taxes at different rates.
* Adapt the XML generation logic to check for product lines with a non-standard VAT rate and apply the necessary modifications. Ensure the final totals in the XML align with Odoo's computed tax amounts.

Task latam side: 1330
Task Adhoc side: 52999

Forward-Port-Of: odoo/enterprise#91392