On older PC(s) the runtime protection can be truly noticeable as anti-virus install hooks/watchdogs on most common functions used by malware. Thus with all the extra services they run to keep your PC safe from virus, can make it quite heavy in terms of resources. ( I believe, AVAST did nearly 32 hooks, imagine having a single process processing all the calls that all the processes do to those particular functions ).I've win 10 and I haven't noticed that it slows anything down.
Nowadays, if you do not execute anything at all but browse, you should be safe, the sandbox of modern browsers is brilliant. However you should never allow sites to execute flash, it is just bad, you can get easily infected with flash no matter the browser you have. A customer of us recently got infected with a miner when browsing a page, after enabling flash, even on latest chrome, they managed to execute the binary on his machine.