iCloud & iCloud Photos · Troubleshooting iCloud space
Will turning on iCloud Drive or iCloud Photos slow down my Mac?
Short answer: Only during the initial catch-up, when large uploads or downloads and Photos' background analysis use network, disk and processing. Afterwards iCloud mostly idles. Leave the Mac plugged in overnight for big syncs and pause Photos when you need bandwidth.
Only while it catches up. The first sync — uploading a large library or downloading files onto a new Mac — uses network, disk and some processing for hours or days, and Photos analyses your library in the background. Once that's done, iCloud mostly sits idle and syncs small changes.
To keep things smooth during a big sync, leave the Mac plugged in and awake overnight, and pause Photos syncing if you need full bandwidth — see pausing iCloud Photos. Storage Studio keeps its own background work light in the same way.