As for speed, since you have the metadata currently auto-generating data on the fly, and you probably have tons of product in one category, it might take a few seconds more.
Category metadata is not based on just what is shown one one page, its based on category description, parent category info, and ALL products within that category (not just what is currently shown). So it's takes into account names of all products in that category, which if you have 100s of them, I can see how this might slow things down a bit.
You shouldn't have this problem for products, as it only takes into account that product, and the assigned category name/description.
To alleviate the category problem, you will want to turn on the "Turn On Manual Edit of Meta Title/Description/Keywords (Global)" setting.
When you do this, instead of generating Title, Description, and Keywords on the fly on the front end, it will refer to the database entry for metadata for that category. But in order to have a database entry for category metadata, you must enter each category via VM admin, and edit/save what is auto-generated there. Now instead of dynamically creating metadata keywords (most intensive process) on the fly as visitors go to your category, the info will be pulled from the database, greatly reducing the processing.
So, if you have follow that.
1. DO NOT TURN ON for products:
Turn On Manual Edit of Meta Title/Description/Keywords (Global)
unless you want to edit (title, description, keywords) for each product. 10,000 products... that is a lot of time.
2. TURN ON for categories:
Turn On Manual Edit of Meta Title/Description/Keywords (Global)
and enter each category in VM admin, then save it. This will save the dynamic info into database, so not having to do this when user visits site front end.
3. The other global settings you can use a s you please... they should not make much difference at all in terms of speed (appending, pre-pending options)