ChiselSim tests runs orders of magnitude slower than chiseltest

[carlosedp]

Converting some tests to ChiselSim, I've noticed the tests run about 20x slower than chiseltest.

As a comparison, running on ChiselSim:

❯ time ./mill chiselv.test.testOnly chiselv.ALUSpec
[86/86] chiselv.test.testOnly
ALUSpec:
- should ADD
- should ADDI
- should SUB
- should AND
- should ANDI
- should OR
- should ORI
- should XOR
- should XORI
- should SRA
- should SRAI
- should SRL
- should SRLI
- should SLL
- should SLLI
- should SLT
- should SLTI
- should SLTU
- should SLTIU
- should EQ
- should NEQ
- should GT
- should GTU
Run completed in 1 minute, 37 seconds.
Total number of tests run: 23
Suites: completed 1, aborted 0
Tests: succeeded 23, failed 0, canceled 0, ignored 0, pending 0
All tests passed.
./mill chiselv.test.testOnly chiselv.ALUSpec  0.99s user 0.79s system 1% cpu 1:40.39 total

and on chiseltest:

❯ time ./mill chiselv.test.testOnly chiselv.ALUSpec
[86/86] chiselv.test.testOnly
ALUSpec:
- should ADD
- should ADDI
- should SUB
- should AND
- should ANDI
- should OR
- should ORI
- should XOR
- should XORI
- should SRA
- should SRAI
- should SRL
- should SRLI
- should SLL
- should SLLI
- should SLT
- should SLTI
- should SLTU
- should SLTIU
- should EQ
- should NEQ
- should GT
- should GTU
Run completed in 3 seconds, 661 milliseconds.
Total number of tests run: 23
Suites: completed 1, aborted 0
Tests: succeeded 23, failed 0, canceled 0, ignored 0, pending 0
All tests passed.
./mill chiselv.test.testOnly chiselv.ALUSpec  0.31s user 0.10s system 8% cpu 4.846 total

I've followed the migration guide which ended with the following simple changes:

❯ gdpatch chiselv/test/src/ALUSpec.scala
diff --git a/chiselv/test/src/ALUSpec.scala b/chiselv/test/src/ALUSpec.scala
index ac56a81..b4bafd0 100644
--- a/chiselv/test/src/ALUSpec.scala
+++ b/chiselv/test/src/ALUSpec.scala
@@ -1,7 +1,7 @@
 package chiselv

 import chisel3._
-import chiseltest._
+import chisel3.simulator.EphemeralSimulator._
 import com.carlosedp.riscvassembler.ObjectUtils.NumericManipulation
 import org.scalatest._

@@ -9,7 +9,7 @@ import Instruction._
 import flatspec._
 import matchers._

-class ALUSpec extends AnyFlatSpec with ChiselScalatestTester with should.Matchers {
+class ALUSpec extends AnyFlatSpec with should.Matchers {
   val one        = BigInt(1)
   val max        = (one << 32) - one
   val min_signed = one << 32 - 1
@@ -124,12 +124,12 @@ class ALUSpec extends AnyFlatSpec with ChiselScalatestTester with should.Matcher
     dut.io.a.poke(i.to32Bit)
     dut.io.b.poke(j.to32Bit)
     dut.clock.step()
-    dut.io.x.peekInt() should be(out)
+    dut.io.x.peek().litValue should be(out)
   }
   def testCycle(
       op: Type
     ) =
-    test(new ALU) { c =>
+    simulate(new ALU) { c =>
       cases.foreach { i =>
         cases.foreach { j =>
           testDut(i, j, aluHelper(i, j, op).to32Bit, op, c)

The file is from https://github.com/carlosedp/chiselv/blob/main/chiselv/test/src/ALUSpec.scala

Type of issue: Bug Report

Please tell us about your environment:

Chisel 6.4.0 on MacOS Sonoma 14.5. Verilator 5.024 2024-04-05 rev UNKNOWN.REV