Huge performance difference: PowerShell Core vs. 5.1

Recently I have answered another question about performance. A solution found using PowerShell-ISE successfully, and good speed testified using PowerShell Core.

However, running the same script(s) in PowerShell 5.1 displays utterly different results, absolutely worse in the performance improvement. Moreover, the original script runs more than 4× slower than in ISE.

So my question is: where lays a reason for such an enormous variance, and what should I avoid in PowerShell scripting (or, conversely, maybe explicitly load a .NET library or something alike)?

PowerShell-ISE: performance improvement cca 10:1 (good):

.\cr2635wrapper.ps1 -maxLoop 8;$  Host.Name;$  PSVersionTable 
VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{0}2{1}3{2}4{3}5{4}6{5}7" orig.     7       269       756        19,574412 VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{5}2{4}3{3}4{2}5{1}6{0}7" answer    7       284       839         1,911312  Windows PowerShell ISE Host  Name                      Value                   ----                      -----                   PSVersion                 5.1.17763.503           PSEdition                 Desktop                 PSCompatibleVersions      {1.0, 2.0, 3.0, 4.0...} BuildVersion              10.0.17763.503          CLRVersion                4.0.30319.42000         WSManStackVersion         3.0                     PSRemotingProtocolVersion 2.3                     SerializationVersion      1.1.0.1 

PowerShell Core: performance improvement cca 28:1 (excellent):

pwsh -noprofile -command .\cr2635wrapper.ps1 -maxLoop 8;$  Host.Name;$  PSVersionTable 
VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{0}2{1}3{2}4{3}5{4}6{5}7" orig.     7       269       756       22,0310531 VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{5}2{4}3{3}4{2}5{1}6{0}7" answer    7       284       839        0,7800336  ConsoleHost  Name                           Value ----                           ----- PSVersion                      6.2.0 PSEdition                      Core GitCommitId                    6.2.0 OS                             Microsoft Windows 10.0.17763 Platform                       Win32NT PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…} PSRemotingProtocolVersion      2.3 SerializationVersion           1.1.0.1 WSManStackVersion              3.0 

PowerShell 5.1: performance improvement cca 4:3 (very poor):

powershell -noprofile -command .\cr2635wrapper.ps1 -maxLoop 8;$  Host.Name;$  PSVersionTable 
VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{0}2{1}3{2}4{3}5{4}6{5}7" orig.     7       269       756       87,1714765 VERBOSE: maxRange=7, lowCountThreshold=1, operators="+-*/" permutations=4096 formatString="1{5}2{4}3{3}4{2}5{1}6{0}7" answer    7       284       839        64,888286  ConsoleHost  Name                           Value ----                           ----- PSVersion                      5.1.17763.503 PSEdition                      Desktop PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...} BuildVersion                   10.0.17763.503 CLRVersion                     4.0.30319.42000 WSManStackVersion              3.0 PSRemotingProtocolVersion      2.3 SerializationVersion           1.1.0.1