Why does this creates TONS of objects, and what should I do to prevent it?

cache.entries.removeAll {
            val res = now - it.value.first > duration
            if (res) <http://log.info|log.info>("Removing value in cache $name for key ${it.key}")
            res
        }

depends on the context around it, if there are captured locals, for example

It's in kotlin.collections.MutableCollections

I would guess that

now

is what makes it create a capture

The full method

open fun clear() {
        val now = now
        cache.entries.removeAll {
            val res = now - it.value.first > duration
            if (res) <http://log.info|log.info>("Removing value in cache $name for key ${it.key}")
            res
        }
        errors.entries.removeAll {
            val res = now - it.value.first > errorDuration
            if (res) <http://log.info|log.info>("Removing error in cache $name for key ${it.key}")
            res
        }
    }

yes, but a single call to

clear

should create just two instances of lambdas

But if I call it twice I get 4 instances? That may explain it

Of course, you capture

now

into a local variable and use it in lambdas. They have to create an instance and capture value in an anonymous class so it is available to the code inside lambda.