post request on search/query can be super slow

Hi !
I'm having some trouble using the endpoint search/query and I'm looking for some tips and advice to optimize it.

1st problem:
Although 2 graph queries are launched in parallel and waited for by a Task.WhenAll the queries are executed one after the other as if they were put in a queue.

2nd problem:
Requests on the endpoint search/query can be super slow (3s or more) but this is not constant. The same request can take 3s or 400ms

below is an example of the object sent to the endpoint search.
Thanks for your help.

{ "AdditionalData": {}, "BackingStore": { "ReturnOnlyChangedValues": false, "InitializationCompleted": true }, "Requests": [ { "AdditionalData": { "trimDuplicates": true }, "AggregationFilters": null, "Aggregations": null, "BackingStore": { "ReturnOnlyChangedValues": false, "InitializationCompleted": true }, "ContentSources": null, "EnableTopResults": null, "EntityTypes": [ 2 ], "Fields": [ "id", "name", "lastModifiedDateTime", "lastModifiedBy", "webUrl", "createdBy", "parentReference", "remoteItem" ], "From": 0, "OdataType": null, "Query": { "AdditionalData": {}, "BackingStore": { "ReturnOnlyChangedValues": false, "InitializationCompleted": true }, "OdataType": null, "QueryString": "createdby:\"Nestor Wilke\" OR createdby:\"Allan Deyoung\" OR createdby:\"Christie Cline\" OR createdby:\"MOD Administrator\" OR createdby:\"Adele Vance\" OR createdby:\"Diego Siciliani\" OR createdby:\"Brian Johnson (TAILSPIN)\" OR createdby:\"Robert Smitho\" OR createdby:\"Quentin PLA\" AND filetype:(docx OR doc OR xlsx OR xls OR xlsm OR pptx OR ppt OR form OR pdf OR jpg OR png OR heic OR heif OR jpeg OR gif OR tiff OR raw OR svg OR bmp OR mp4 OR avi OR mov OR webm OR mkv OR flv OR wmv OR mpeg OR m4v OR 3gp OR zip OR rar OR tar OR 7z OR gzip OR cab OR one OR onetoc2) AND lastmodifiedtime>2022-01-01 AND lastmodifiedtime<2023-05-05 AND (createdBy<>\"System Account\" AND createdBy<>\"SharePoint App\")", "QueryTemplate": null }, "QueryAlterationOptions": null, "Region": null, "ResultTemplateOptions": null, "SharePointOneDriveOptions": null, "Size": 10, "SortProperties": [ { "AdditionalData": {}, "BackingStore": { "ReturnOnlyChangedValues": false, "InitializationCompleted": true }, "IsDescending": true, "Name": "LastModifiedDateTime", "OdataType": null } ] } ] }

Start by reproducing the supplied search/query request and examine the two graph queries mentioned as running through Task.WhenAll. Measure whether they execute sequentially and when the same request takes 3 seconds versus 400 ms; done means identifying the cause or narrowing it down with reproducible evidence.