I noticed this too. My 2011 MacBook Air loads sites in a split second. Not so all the time on the 12.9" iPad Pro which is apparently faster.
I'm noticing how many web pages are mobile format only on safari. They have a low resolution header banner which makes me leave instantly.
As is said often. Hopefully iOS 10 fixes the issue.
Hold your 'refresh' button down in SmartSearch bar, it'll give you the option for the desktop site. The 12.9" iPP IS faster than your '11 MBA. Go to desktop sites, then scroll, it'll haul ba&^lls! I've both the 12.9 and 9.7" iPPs and they're fast as hell on Safari. Crappy code can f a site up much faster than your hardware though (and in mobile, NO processors are beating the A9x's in Browsing/SnSpdr, et al).
I think a lot of mobile sites are optimized for 'just that', mobile. Grab the desktop site, and you'll find it screams. My 12.9" and 9.7" Pros both nearly match the performance of my 15" rMBP 2.8/16/1TB machine, and it's no slouch. They're certainly the fastest iPads to date (and EVERY iPad version has bested the previous, as well as led the pack for the ½ year before ...once in a blue moon, an Android SoC happens to get close in SS/Octane or a different browsing/CPU workload. the Doubling of RAM in the latest devices, quadrupling in the iPP 12.9 eliminates the Saf tab reload issues from the past.
I was actually filling in a response the other day, forgot or was sidetracked and two days later clicked 'er on, had to do sec code as it was too long a delay for finger, popped up with a ½ dozen tabs open in Safari, all my info filled in as well as response ½ written, exactly in same position the page was when I walked away.
I think you're seeing the crappy coding on some of the mobile presentations of sites. As well, it's not really normal to open a busy site like FB and scroll as fast as possible to race to the bottom. You can go from bottom to top immediately, without 'white spots' or delay by tapping to of display - under clock, regardless of business or 'code issues', you'll be back to the top in a ½ second.
...Assuming you're using Safari and a recent, 2-4GB RAM device.
J