Is it normal for fans to run at 6,000+ rpm at <20% CPU load? I have a 2011 Macbook Pro 15" running Sierra.

The Macbook's fans are annoying me to no end - they are running most of the time I do serious work, right from the minute I start. It can get hot here in the summer, but it happens also at reasonable room temperatures, like right now, at 23°C (73°F).

Here's a graph of my CPU load over an extended period of time, looking fairly relaxed to my eyes:



yet the fan is running constantly:



This has been happening for the past three versions of OS X, so it's nothing version specific. (I started thinking that maybe this isn't normal only recently.)

Things I have tried:


Made sure there are no processes running in activity monitor that hog a lot of CPU time (my work tends to be CPU/GPU intensive though, Photoshop, IDEs, etc. open in parallel all the time)
Reset NVRAM and SMC, many times
Opened the device and removed a giant piece of lint from behind the air ducts (I was sure that'd fix it):



(I didn't do anything else in there other than remove the easily accessible, obvious blockage, though)


Is this normal? Is there anything I can do? Is there some unofficial way to get to the CPU, and maybe apply some better thermal paste? Is there some other potential bottleneck that I should look at other than CPU?

The machine is perfectly usable for me otherwise despite its age (after a RAM and SSD upgrade).
You need to strip &amp; clean the entire airflow, if that fluff-ball was clogged in the outlet, there's an entire duvet inside. All the machines here get a full strip &amp; clean once a year, with a 'quick blow round' in between. If it's gone 6 years without cleaning, it's likely to be pretty well-insulated in there by now.

iFixit teardowns are second to none - there is far too much for me to try to précis here
iFixit - MacBook Pro 15" Unibody Early 2011 Teardown