In order to query the servicenow incident records and to list out the required fields use the sysparm_fields column which helps in optimization. Now,  See the code, result and performance

All Fields

function Get-ServiceNowIncident {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)]
        $IncidentID,

        [Parameter(Mandatory)]
        [pscredential]
        $Credential
    )
    try {
        $Uri = "https://dev42835.service-now.com/api/now/table/incident?sysparm_query=number=$($IncidentID)"
        $Result = Invoke-RestMethod -Uri $Uri -Method Get -Credential $Credential
        $Result.result | ConvertTo-Json
    }
    catch {
        $_.Exception
    }
}

Measure-Command -Expression {Get-ServiceNowIncident -IncidentID INC0000059 -Credential admin}

Here is the result! (8 seconds for one record)

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 8
Milliseconds      : 468
Ticks             : 84680174
TotalDays         : 9.80094606481481E-05
TotalHours        : 0.00235222705555556
TotalMinutes      : 0.141133623333333
TotalSeconds      : 8.4680174
TotalMilliseconds : 8468.0174

Selected Fields!

function Get-ServiceNowIncident {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)]
        $IncidentID,

        [Parameter(Mandatory)]
        [pscredential]
        $Credential
    )
    try {
        $Uri = "https://dev42835.service-now.com/api/now/table/incident?sysparm_query=number=$($IncidentID)&sysparm_fields=number,state,opened_at&sysparm_limit=1"
        $Result = Invoke-RestMethod -Uri $Uri -Method Get -Credential $Credential
        $Result.result | ConvertTo-Json
    }
    catch {
        $_.Exception
    }
}

Measure-Command -Expression {Get-ServiceNowIncident -IncidentID INC0000059 -Credential admin}

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 6
Milliseconds      : 540
Ticks             : 65405344
TotalDays         : 7.57006296296296E-05
TotalHours        : 0.00181681511111111
TotalMinutes      : 0.109008906666667
TotalSeconds      : 6.5405344
TotalMilliseconds : 6540.5344

It’s 6 seconds! Approximately we get 2 seconds per record. Good Sign! And see how it impacts if you select with pipe line for all fields.

Get-ServiceNowIncident -IncidentID INC0000059 -Credential admin | Select number , state , opened_at

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 10
Milliseconds      : 540
Ticks             : 65405344
TotalDays         : 7.57006296296296E-05
TotalHours        : 0.00181681511111111
TotalMinutes      : 0.109008906666667
TotalSeconds      : 6.5405344
TotalMilliseconds : 6540.5344

Tip: Use the filter parameters as effective as possible.