Issue #18293 has been updated by Eregon (Benoit Daloze).


I think it'd be good to understand the slow down here?

Is it due to having the method in Ruby and using `Primitive.`?
It seems unexpectedly high for that.

Or maybe the fact there is a declared keyword argument is slow if the kwarg is not given?
But that's not new, e.g. `read_nonblock` uses this pattern since a long time.

----------------------------------------
Bug #18293: Time.at in master branch was 25% slower then Ruby 3.0
https://bugs.ruby-lang.org/issues/18293#change-94540

* Author: watson1978 (Shizuo Fujita)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-11-08T13:15:21Z master bd2674ad33) [arm64-darwin21]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
       | Ruby 3.0.2 | Ruby 3.1.0-dev | result
--       | --         | --             | --
Time.at  | 8.223M     | 6.190M         | 0.753x

### Environment
- MacBook Pro (14inch, 2021)
- macOS 12.0
- Apple M1 Max
- Apple clang version 13.0.0 (clang-1300.0.29.3)

### Ruby 3.1.0-dev
```
$ ruby -v time.rb
ruby 3.1.0dev (2021-11-08T13:15:21Z master bd2674ad33) [arm64-darwin21]
Warming up --------------------------------------
             Time.at   614.843k i/100ms
Calculating -------------------------------------
             Time.at      6.190M ( 0.3%) i/s -     31.357M in   5.065559s
```

### Ruby 3.0.2
```
$ ruby -v time.rb
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [arm64-darwin21]
Warming up --------------------------------------
             Time.at   821.722k i/100ms
Calculating -------------------------------------
             Time.at      8.223M ( 0.6%) i/s -     41.908M in   5.096820s
```

### Test code
```ruby
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report('Time.at') { Time.at(0) }
end
```



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