Header bidding can quietly wreck a page: every client-side bidder is more JavaScript and another connection, and the auction timeout sits right in the path of the user seeing content. Done carelessly, you trade CPM for bounce.

The costs

• Connection limits. A browser can only open so many connections at once; each client-side bidder spends some.
• Blocking load. Scripts that load synchronously delay the page from rendering.
• Timeout effect on Core Web Vitals. A long auction delays the ad and can hurt LCP and shift layout (CLS).
• Too many client-side bidders. Each one adds weight whether or not it ever wins.

The mitigations

• Trim bidders that rarely or never win.
• Move the long tail to s2s (Track E) to cut page weight.
• Adaptive timeouts, shorter on slow devices and networks.
• Lazy / viewport auctions: only run the auction for slots near the viewport.
• A failsafe, so a stalled auction never blocks the ad forever.