ping response time charts

For the last few days I've been graphing ping response time to my TP-Link Archer AR900 running over BT Infinity. Sync is around 35/6 making max thruput around 32/5. I have graphs from two locations

Here's an example
https://f8lure.mouselike.org/planetf1.ddns.net_small...

The first part of this graph is where an online backup was occuring, though the app (crashplan) is set to max out at around 4500 Mbps so in theory my connection shouldn't be swamped. The latter part is after the backup completes.

A few questions
* Is this typical? It surprised me
* Even in low traffic periods I see some delays. Any reason to doubt the router vs the connection/business as usual?

I should add no specific router QOS rules have been set. I can do this on host/port. I know on the asus this disables hw accel, no idea if the case on the archer, though my internal network speeds aren't too high anyway

Maybe it is the processing power of the router - if the router is busy then it may lower the priority of responding to pings because it has "real" work to do.

Tricky to know without wireshark or similar. A reminder that I'm more comfortable with that, kernel traces & iptables than I am in trying to second guess appliances/services....

Been looking at the QOS rules, but the router support is limited, without going ddwrt or very upmarket. former loses some performance perhaps.