Garbage collection is normal and will always happen. Its threshhold is something the game determines based on memory use and previous garbage collections.
20k/sec is fairly bad but should be tolerable. I edited the titan performance plugin to always display kb/sec, and mine idles around .5k standing still doing nothing, goes to around 1.5k when running around and solo fighting, and occasionally peaks around 5k. (Groups are getting annoyed at me stopping when it breaks 2k/sec and I try to find out why lol)
You can increase your AddOn memory to make the garbage collection happen less frequently, but don't crank it up too high or when the garbage collection hits it's going to make the pauses longer. 48MB is probably a good first bump if you're at 32MB like most.
Best solution is to identify the problem mods and remove/replace them. LittleJohn wrote a handy mod called
TraceEvent you can use to identify the problem mods. Be aware that examining the data can alter it. So I'd use it to find what mods are really busy or creating a lot of garbage and then try removing the mod and seeing what youre kb/sec goes down to.
If your garbage collection climbs at 20k/sec while idle (I hope not), you have some mods really abusing the OnUpdate event. Using TraveEvent, enter "OnUpdate$" without the quotes and hit Shift+Add, then hit Shift+Sort and it will display a list of mods' OnUpdates in order of UI memory increases.
But use it as a guide. In the examining of the data it can skew things wildly. Like a table.sort using another function may in actuality create 512 bytes of garbage, but with TraceEvent watching the flurry of table.sort it could appear to create 250k bytes. Remove the mod, reload the UI and see if memory use went down a lot.
Remember to backup SavedVariables when temporarily removing/experimenting with mods too. If a mod doesn't exist when the UI loads it will dump all the SavedVariable information for that mod.