And I completely understand "here's a nickle kid, get a real computer" 
I mostly use the Pi for sensors that are already in place. Some are in bad locations to hook a real laptop to and since the Pi can run fine off an external cell phone battery I don't even need AC handy which is nice.
So I tried to get clever. I moved arduino-builder to arduino-builder.real, made arduino-builder a script which adds '-jobs 4' to the parameters passed.
Output of arduino-builder help:
-jobs int
specify how many concurrent gcc processes should run at the same time. Defaults to the number of available cores on the running machine
I tried both -jobs=4 and -jobs 4. neither threw a syntax error but neither limited the concurrent builds.
root@pi4:/opt/arduino-1.8.9# cat arduino-builder
#!/usr/bin/perl
print "calling arduino-builder -jobs 4 @ARGV\n";
system "/opt/arduino-1.8.9/arduino-builder.real -jobs 4 @ARGV";
When I compile I see:
calling arduino-builder -jobs 4 -compile -logger=machine -hardware /opt/arduino-1.8.9/har....
I also added a 8G USB stick as device swap and hoped that would get it over the hump building the initial core. The pi didn't crash then but it hung tight enough swapping to drop the network connection and so IDE session I was tossing. I'll add vncserver and run it local, that might get it.
Reverting to 1.8.5 works as well.
Cheers!
Lee