The trouble with meta queries is they require an additional join per filter. Say you have a property site, and you're searching by location, rooms and price. That's three joins. If you were using taxonomies, just two (
term_taxonomy) - no matter how many filters.
The other reason taxonomies tend to beat meta queries is their db schema. They're well indexed and optimised for searching - the meta table, whilst indexed on meta keys, has a value field that's large text - start querying on that bad boy (especially
LIKE statements) and MySQL will have to take a breath or two.
Having said all that, meta queries have their place and can be extremely useful (with caution). And to be honest, unless you've got thousands of posts, or you're applying several meta queries, the difference will probably be negligible.