How the WebRequest API works is:

1. Network process downloads data
2. Sent to the WebExtension (delay #1)
3. Extension runs arbitrary JavaScript (Slow to very slow)
4. Results eventually make it to the WebProcess where it belongs.

This is slow and will always be. Their change to remove blocking turns step 2/3 into a copy of the data, not a blocking call.

Now an ad can be slower, just by more data or bad JS. But that isn’t Googles concern because they sell those ads.