Will multi-store make my Magento slower?

Done right, no. The Magento application is shared in memory; each store view just changes what gets rendered. Done wrong, with bloated core_config_data, runaway URL rewrites per store, or unscoped extension queries, multi-store can be measurably slower. We size Redis + Varnish for the storefront count, scope every config to the right level, and load-test each store view separately before launch. We commit Lighthouse 90+ on every storefront, regardless of count.